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THE INTERNATIONAL SCIENTIFIC SERIES. 

VOLUME XXXIV. 



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I 



New York: D. APPLETON & CO., 72' Fifth Avenue. 



X 



LX 



TELE INTERXATIONAL SCIENTIFIC SERIES 



THE SUN 



^ 






BY 



C. A. YOUNG, Ph.D., LL.D. 

PROFESSOR OF ASTRONOMY IN PRINCETON UNIVERSITY 



WITH NUMEROUS ILLUSTRATIONS 



NEW AND REVISED EDITION 



NEW YORK 
D. APPLETON AND COMPANY 



1895 



— fw 



Copyright, 1881, 1886, 1895, 
By D. APPLETON AND COMPANY. 



PREFACE TO THE EEYISED EDITION. 



Since the original publication of this book in 1881 
great advances have been made in our knowledge of the 
sun, and in the four or five editions which have subse- 
quently appeared the attempt has been made to keep 
the book measurably up to date by the addition of 
appendices and notes. 

The time has come, however, when such expedients 
are no longer adequate, and the author has therefore 
thoroughly revised the work, rewriting portions, em- 
bodying notes in the text, and adding whatever seemed 
necessary to make the book fairly representative of the 
solar science of to-day. 

The j)rogress of discovery with respect to helium 
has been so continuous and rapid during the revision 
and printing of the work, that I have found it neces- 
sary to append a supplementary note upon the subject. 

Special thanks are due to Prof. Hale for several of 
the finest of the twenty new illustrations, and to Ginn 
and Co. for the use of one or two cuts from my Gen- 
eral Astronomy. 

November, 1895, 



FKOM PKEFACE TO THE FIEST EDITION. 



It is my purpose in this little book to present a gen- 
eral view of what is known and believed about the sun, 
in language and manner as unprofessional as is con- 
sistent with precision. I write neither for scientific 
readers as such, nor, on the other hand, for the masses, 
but for that large class in the community who, without 
being themselves engaged in scientific pursuits, yet 
have sufl[icient education and intelligence to be inter- 
ested in scientific subjects when presented in an un- 
technical manner ; who desire, and are perfectly com- 
petent, not only to know the results obtained, but to 
understand the principles and methods on w^hich they 
depend, without caring to master all the details of the 
investigation. 

I have tried to keep distinct the line between the 
certain and the conjectural, and to indicate as far as 
possible the degree of confidence to be placed in data 
and conclusions. 

It is hardly necessary to say that the work has small 
claims to originality. I have made use of material 
suited to my purpose from all accessible sources ; possi- 



FROM PUEFACE TO THE FIRST EDITION. yii 

biy in some cases (though I liope not) without giving 
sufficient credit to the original authority. I have been 
specially indebted to Secchi, Lockyer, Proctor, Ean- 
yard, Yogel, Schellen, and Langle3\ . . . 

Princeton, August i, 1881, 



CONTENTS. 



I^^TRODUCTION. 

PAGE 

The Sun's Eelation to Life and Activity upon the Earth.— Brief State- 
ment of the Principal Facts relating to the Sun, and of the Ac- 
cepted Views as to its Constitution 1 



CHAPTER I. 

DISTANCE AND DIMENSIONS OF THE SUN. 

Importance of the Problem. — Definition of Parallax. — Aristarchus's 
Determination of the Parallax. — Different Available Methods. — 
Observations of Mars and of the nearer Asteroids. — Transits of 
Venus. — Observations of Contacts, Heliometric Measures, and 
Photographic Work. — Determination of Solar Parallax by means 
of the Velocity of Light ; by Lunar and Planetary Perturbations. — 
Illustrations of the Immensity of the Sun's Distance. — Diameter 
of the Sun, — The Sun's Mass and Density . . ... 10 



CHAPTER IL 

METHODS AND APPARATUS FOR STUDYING THE SUR- 
FACE OF THE SUN. 

Projection of Solar Image upon a Screen. — Carrington's Method of 
determining the Position of Objects on Sun's Surface. — Solar 
Photography. — Photoheliographs. — Cornu's Methods. — Telescope 
with Silvered Object-Glass. — Herschel's Solar Eyepiece. — Tlie 

Polarizing Eyepiece 43 

ix 



X CONTENTS. 

CHAPTEK III. 
THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 

PAGE 

The Spectrum and Fraunhofer's Lines.— The Prismatic Spectroscope ; 
Description of Various Forms and Explanation of its Operation. — 
The Dittraction Spectroscope. — The Concave Grating. — Analyzing 
and Integrating Spectroscopes.— The Telespectroscope and its 
Adjustment. — The Spectrograph.— Explanation of Lines in the 
Spectrum.— Kirchhoff's Researches and Laws.— The Sun's Ab- 
sorbing Atmosphere and Eeversing Layer.— Elements present in 
the Sun. — Lockyer's Eesearches and Hypothesis.— Basic Lines. — 
Dr. H. Draper's Investigations as to the Presence of Oxygen in 
the Sun.— Schuster's Observations.— Effect of Motion upon Wave- 
Length of Rays and Spectroscopic Determinations of Motion in 
Line of Sight 59 

CHAPTER IV. 

SUN-SPOTS AND THE SOLAR SURFACE. 

Granulation of Solar Surface. — Views of Langley, Nasmyth, Secchi, 
and others. — Faculae. — Nature of the Photosphere. — Janssen's 
Photographs of Solar Surface — the Reseau Photospherique. — Dis- 
covery of Sun-Spots. — General Appearance and Structure of a 
Spot. — Its Formation and Disappearance. — Duration of Sun-Spots. — 
Remarkable Phenomena observed by Carrington and Hodgson. — 
Observations of Peters. — Dimensions of Spots. — Proof that Spots 
are Cavities. — Sun-Spot Spectrum. — " Veiled Spots." — Rotation of 
Sun. — Equatorial Acceleration. — Explanations of the Accelera- 
tion. — Position of Sun's Axis and Secchi's Table for its Position 
Angle at Different Times in the Year. — Proper Motions of Spots. — 
Distribution of Spots 102 



CHAPTER V. 

PERIODICITY OF SUN-SPOTS; THEIR EFFECTS UPON THE 
EARTH, AND THEORIES AS TO THEIR CAUSE AND NA- 
TURE. 

Observations of Schwabe. — Wolf's Numbers. — Proposed Explanations 
of Periodicity. — Connection between Sun-Spots and Terrestrial 
Magnetism. — Remarkable Solar Disturbances and Magnetic 



d 



■ 



CONTENTS. XI 



Storms. — Effect of Sun-Spots on Temperature. — Sun-Spots, Cy- 
clones, and Kainfall. — Kesearches of Symons and Meldrum. — Sun- 
Spots and Commercial Crises. — Galileo's Theory of Spots. — Her- 
schel's Theory. — Secchi's First Theory. — Zollner's. — Faye's. — 
Secchi's Later Opinions. — Theories of Lockyer, Schaeberle, Oppol- 
zer, and others 151 



CHAPTER VI. 

THE CHROMOSPHERE AND THE PROMINENCES. 

Early Observations of Chromosphere and Prominences. — The Eclipses 
of 1842, 1851, and I860.— The Eclipse of 1868.— Discovery of Jans- 
sen and Lockyer. — Arrangement of Spectroscope for Observations 
upon Chromosphere. — Spectrum of Chromosphere. — Lines always 
present. — Lines often reversed. — Spectrographs of Hale and Des- 
landres. — Motion Forms. — Double Reversal of Lines. — Distribution 
of Prominences. —Magnitude. — Classification of Prominences as 
quiescent, and eruptive or metallic. — Isolated Clouds. — Violence 
of Motion. — Observations of August 5, 1872. — Theories as to the 
Formation and Causes of the Prominences 192 



CHAPTER VII. 

THE CORONA. 

General Appearance of the Phenomenon. — Various Representations.— 
Eclipses of 1857, 1860, 1867, 1868, 1869, 1871, and 1878.— Proof that 
the Corona is mainly a Solar Phenomenon. — Brightness of the 
Corona. — Connection with Sun-Spot Period. — Spectrum of the 
Corona. — Application of the Analyzing and Integrating Spectro- 
scopes. — Polarization. — Evidence of the Slitless Spectroscope as 
to the Constitution of the Corona.— Changes and Motions in the 
Corona. — Its Form and Constitution, and Theories as to its Nature 
and Origin 237 



CHAPTER VIIL 

THE SUN'S LIGHT AND HEAT 

Sunlight expressed in Candle- Power. — Method of Measurement.— 
Brightness of the Sun's Surface.— Langley's Experiment.— Dimin- 



xii CONIENTS. 



PAGE 

ution of Brightness at Edge of the Sun's Disk.— Hastings's View 
as to Nature of the Absorbing Envelope.— Total Amount of Ab- 
sorption by Sun's Atmosphere.^Thermal, Luminous, and Actinic 
Kays ; their Fundamental Identity and Differences. — Measurement - 
of the Sun's Eadiation. — Herschel's Method.— Expressions for the 
Amount of Sun's Heat — Pouillet's Pyrheliometer. — Crova's. — 
Violle's Actinometer. — Langley's Researches. — Absorption of Heat 
by Earth's Atmosphere : by the Sun's. — Question as to Differences 
of Temperature on Different Portions of Sun's Disk. — Question as 
to Variation of Sun's Eadiation with Sun-Spot Period. — The Sun's 
Temperature — Actual — Effective.— Views of Secchi, Ericsson, Pou- 
illet,Vicaire, Kosetti, Le Chatelier, and Wilson and Gray. — Evidence 
from the Burning-Glass. — Langley's Experiment with the Besse- 
mer " Converter." — Permanency of Solar Heat for last Two Thou- 
sand Years. — Meteoric Theory of Sun's Heat. — Helmholtz's Con- 
traction Theory. — Possible Past and Future Duration of the Sun's 
Supply of Heat. — Siemens's Untenable Theory .... 276 



CHAPTEE IX. 

SUMMARY OF FACTS, AND DISCUSSION OF THE CONSTI- 
TUTION OF THE SUN, 

Table of Numerical Data. — Constitution of Sun's Nucleus. — Peculiar 
Properties of Gases under High Temperature and Pressure. — 
Characteristic Differences between a Liquid and a Gas. — Consti- 
tution of the Photosphere and Higher Eegions of the Sun's At- 
mosphere. — Professor Hastings's Theory. — Pending Problems of 
Solar Physics . 823 

Note on Helium 344 

Index 351 



I 



THE STJ^. 



INTKODUCTION. 

TRE sj7:n''s relation to life and activity upon the earth^ 

Brief Statement of the Principal Facts relating to the Sun, and of the 
Accepted Views as to its Constitution. 

It is true that from the highest point of view the 
sun is only one of a multitude — a single star among 
millions — thousands of which, most likely, exceed him 
in brightness, magnitude, and power. He is only a pri- 
vate in the host of heaven. 

But he alone, among the countless myriads, is near 
enough to affect terrestrial affairs in any sensible degree ; 
and his influence upon them is such that it is hard to 
find the word to name it ; it is more than mere control 
and dominance. He does not, like the moon, simply 
modify and determine certain more or less important 
activities upon the surface of the earth, but he is almost 
absolutely, in a material sense, the prime mover of the 
whole. To him we can trace directly nearly all the 
energy involved in all phenomena, mechanical, chemi- 
cal, or vital. Cut off his rays for even a single month, 
and the earth would die ; all life upon its surface would 
cease. 

There always has been a more or less distinct recog- 
nition of this fact. The first man's experience of the 
2 



2 INTRODUCTION. 

first sunset ever witnessed by human eyes must have 
made it tremendously obvious, when he saw the sun 
descend below the horizon, and the darkness close inj 
upon the earth, and felt the chill of night, and felli 
asleep not knowing of a sunrise to come — unless, per- 
haps, some divine revelation took pity on the hopeless 
terror he must otherwise have suffered, or unless he 
may have been, like a little child, slow to notice and 
unable to comprehend what would frighten a more in- 
telligent being. 

But while the material supremacy of the sun has 
always been recognized by thoughtful minds, and has 
even been made the foundation of religious systems, as 
with the Persians, it has been reserved for more mod- 
ern times, and to our own century, to show clearly just 
liow, in what sense, and how far the sunbeams are the 
life of the earth, and the sun himself the symbol and 
vicegerent of the Deity. The two doctrines of the corre- 
lation of forces and the conservation of energy, having 
once been distinctly apprehended and formulated, it 
has been comparatively easy to confirm them by experi-. 
ment and observation, and then to trace, one by one, to I 
their solar origin, the different classes of energy which* 
present themselves in terrestrial phenomena — to show, 
for instance, how the power of waterfalls is only a trans- 
formation of the sun's heat ; and that the same thing 
is true, a little more remotely but just as certainly, of 
the power of steam, of electricity, and even of animals. 
The idea is now so famiUar that it is hardly necessary 
to dwell upon it, and yet, for some of our readers at 
least, it may be worth while to examine it a little more 
closely. 

Whenever work is done, it is by the undoing of some 
previous work. When a clock moves, it is the unwind- 



INTRODUCTION. 3 

ing of a spring or the falling of a weight which keeps 
it going, and some one mnst have wonnd it up to begin 
with. If the water of a river falls year after year over 
a cataract, and is intercepted to drive our mill-wheels, 
the river continues to run because some power is con- 
tinually raising and returning to the hill-tops the water 
which has flowed into the sea — a process precisely 
equivalent to the daily rewinding of the clock. If the 
powder in a rifle explodes and drives out the bullet, its 
explosive energy depends upon the fact that some power 
has placed the component molecules in such relations 
that, when the trigger is pulled, and the exciting spark 
has, so to speak, cut the bonds which hold them apart, 
they rush together just as suspended weights would 
fall if freed. Before the same substance, which once 
was a charge of gunpowder, but now is dust and gas, 
can again do the same work, the products of the ex- 
plosion must by some power be decomposed, and the 
atoms replaced in the same relations as before the firing 
of the gun ; and this process is mechanically analogous 
to the lifting of fallen weights and placing them upon 
elevated shelves, or hanging them from hooks, ready to 
drop again when the occasion may require. 

Precisely the same thing is true of the heat pro- 
duced by the combustion of ordinary fuel : it is due to 
the collapse of molecules, for the most part of oxygen 
on one side, and carbon and hydrogen on the other, 
which have been separated and built up into structures 
by the action of some laboring power. 

The same can be said of animal power, for all inves- 
tigation goes to show that in a mechanical sense the 
body of an animal is only a very ingenious and effective 
machine, by means of which the living inhabitant which 
controls it can utilize the energy derived from the food 



4 INTRODUCTION. 

taken into the stomach. The body, regarded as a mech- 
anism, is only a food-engine in whicli the stomach and 
hmgs stand for the furnace and boiler of a steam-engine, 
the nervous system for the valve-o-ear, and the muscles 
for the cylinder. How the personality within, which 
wills and acts, is put into relation with this yalve-gear, 
so as to determine the moyements of the body it re- 
sides in, is the inscrutable mystery of life ; the facts 
in the case, however, being no less facts because inex- 
plicable. 

And now, when we come to inquire for the source 
of the energy which lifts the water from the sea to the 
mountain-top, which decomposes the carbonic acid of 
the atmosphere, and plant-foods of the soil, and builds 
up the hydrocarbons and other fuels of animal and 
vegetable tissue, we tind it always mainly in the solar 
rays. I say mainly because, of course, the light and 
heat of the stains, the impact of meteoi^s, and the prob- 
able slow contraction of the earth, are all real sources of 
energy, and contribute their quota. But, as compared 
with the energy derived from the sun, their total 
amount is probably something like the ratio of starlight 
to sunlight ; ^ so small that it is quite clear, as we said 

* PouUlet, about ISoS, came to a conclusion entirely inconsistent 
with the statement of the text. From his actinometrie observations, 
he deduced a temperature of —224' F. (—142° C.) for the "tem- 
j>oratuivof space," which is 236'' (131 C.) above the absoUite zero. To 
maintain this temperature of — 224 *", he calculated that the stars and 
^pace in general must furnish to the earth about 85 per cent, as much 
heat as the sun supplies. His calculations, however, rest upon assump- 
tions as to the laws of cooling and radiation which are not at present re- 
ceived as accurate, and he fails to take proper account of the influence of 
water-vapor in the air — an influence, the magnitude of which was first 
brought out more than twenty years later by the researches of Tyndall 
and Magnus. It is now generally admitted, therefore, that his result can 
not be accepted. 



•I 



'I 



INTRODUCTION. 5 

before, that a month's deprivation of the solar rays 
would involve the utter destruction of all activity upon 
the earth. 

It is natural, therefore, that modern science should 
make much of the sun, and that the study of solar phe- 
nomena and relations should be pursued with the great- 
est interest. For the last fifty years this has been 
especially the case : Schwabe's discovery of the perio- 
dicity of the sun-spots in 1851 ; the development of 
spectroscopic analysis between 1854 and 1870; the 
eclipse observations since 1860 ; the researches of Car- 
rington, Huggins, De La Rue, Lockyer, Janssen, Secchi, 
Vogel, Langley, Hale, and others ; the establishment of 
the observatories at Potsdam and Meudon — these are 
all evidences of the ardor with which astronomers have 
devoted themselves to the problems of solar science, and 
of their rich rewards. 

It may be well, before entering upon the more 
extended discussion of our subject, to summarize here 
a few of the more important and obvious facts re- 
lating to the sun, with a brief statement of the views 
at present generally held in regard to its constitu- 
tion. 

To the few unaided eyes w^iich are able to bear 
its brilliance without flinching, the sun presents the 
appearance of a round, white disk, a little more than 
half a degree in diameter — i. e., a row of seven hundred 
suns, side by side, would just about fill up the circle of 
the horizon. Usually, without a telescope, the surface 
appears simply uniform, except that there is a slight 
darkening at the edge, and that once in a while black 
spots are seen upon the disk. There is nothing in the 
sun's appearance to indicate his real distance, and, until 
that is known, of course no conclusion can be arrived at 



6 INTRODUCTION. 

as to his true dimensions ; but the heat of his rays is 
obvious, and, long before the days of telescopes and 
thermometerSj led to the conclusion that he is nothing 
more or less than an enormous ball of fire. 

If we watch him from day to day through the year, 
beginning about the 21st of March, w^e shall find that 
at noon he daily rises higher in the heavens, until about 
the 22d of June ; at this time he ascends to the same 
height each noon for several successive days, and then 
slides slowly south, passing on September 22d the ele- 
vation he had at starting, and keeping on until, on De- 
cember 21st, he attains his farthest southing ; thence he 
returns, till he reaches the place of beginning, and 
night and day again are equal. 

If, at the same time, one has noticed the stars each 
night, he will find the constellations to have shifted 
with the months, in such a way that it is clear that the 
sun has been traveling eastward among them through 
the sky, as well as swinging north and south ; moving, 
in fact, yearly around the heavens in a path which is a ' 
great circle of the sphere, inclined some 23^° to the 
equator, and called the ecliptic, because it is only when 
the moon is near this line at new or full that eclipses 
happen. 

There is nothing in this motion which of itself can 
inform us whether its cause is a real movement of the 
sun around the earth, or of the earth around the sun. 
At present, of course, every one knows that the earth 
is really the moving body. A careful watching shows 
that her path is not quite circular, or, at least, that the 
sun is not exactly in the center, since it is one hundred 
and eighty-six days through the summer from the ver- 
nal to the autumnal equinox, and only one hundred and 
seventv-nine from tlie autumnal to the vernal. 



INTRODUCTION. 7 

This much was known to the ancients, and the one 
additional fact that the sun's distance is many times 
greater than that of the moon ; it is all that could pos- 
sibly be learned without the use of the telescope and 
instruments of precision. 

Modern astronomy has gone much further. We 
now know that the sun's average distance from the 
earth is about 93,000,000 miles, and consequently that 
his diameter is about 865,000 miles. The sun has 
been weighed against the earth and found to contain 
a quantity of matter nearly 330,000 times as great, and 
comparing this with his enormous bulk, it appears that 
his mean density is only about one fourth that of 
the earth, or one and a quarter times that of water 
— in other words, the mass of the sun is about one 
fourth greater than that of a globe of water of the same 
size. 

The visible surface of the sun has been named the 
photosphere^ and by watching the spots, which occa- 
sionally appear upon it, we have ascertained that it 
revolves upon its axis once in about twenty-five and a 
quarter days. At times of total eclipse, when the moon 
hides from us the body of the sun, we are enabled to 
see certain outlying phenomena at other times invisible. 
"We find close around the luminous surface a rose-col- 
ored stratum of gaseous matter to which Frankland and 
Lockyer some years ago assigned the name of chromo- 
sphere. Here and there great masses of this chromo- 
spheric matter rise high above the general level like 
clouds of flames, and are then known di^ prominences or 
protuberances. 

Outside of the chromosphere is the mysterious co- 
rona^ an irregular halo of faint, pearly light, composed 
for the most part of radial filaments and streamers, 



8 INTRODUCTION. 

which extend outward from the sun to an enormous 
distance ; often more than a million of miles. 

The spectroscope informs us that, in great part at 
least, the elements, which exist in the lower regions of 
the solar atmosphere in the state of vapor, are metals 
we are familiar with upon the earth ; while it shows 
the chromosphere and prominences to consist mainly 
of hydrogen and helium, and makes it possible to ob- 
serve them even when the sun is not hidden by the 
moon. The secret of the corona it fails to unlock as 
yet, though it informs us of the presence in it of an 
unknown gas of inconceivable tenuity. 

The pyrheliometer and actinometer measure for us. 
the outflow of solar heat, and show us that the blaze is 
at least seven or eight times as intense as that of any 
furnace known to art. The quantity of heat emitted is 
enough to melt a shell of ice more than a foot thick 
over the wliole surface of the sun every second of time : 
this is equivalent to the consumption of a layer of the 
best anthracite coal over five inches thick ever^^ single 
minute. 

Combining the facts just stated, astronomers are for 
the most part agreed upon the following conclusions as 
to the constitution of the sun : 

1. The central portion is probably for the most part 
a mass of intensely heated gases. 

2. The photosphere is a shell of luminous clouds, 
formed by the cooling and condensation of the conden- Ij 
sible vapors at the surface, where exposed to the cold 
of outer space. 

3. The chromosphere is composed mainly of uncon- 
densible gases (conspicuously hydrogen) left behind by 
the formation of the photospheric clouds, and bearing 
something the same relation to them that the oxygen. 



INTRODUCTION. 9 

and nitrogen of our own atmosphere do to our own 
clouds. 

4. The corona as yet has received no explanation 
which commands universal assent. It is certainly truly 
solar to some extent, and very possibly may be also to 
some extent meteoric. 



CHAPTER 1. 

DISTANCE AND DIMENSIONS OF THE SUN. 



^1 

eter^ 
;rva- J | 



Importance of the Problem. — Definition of Parallax. — Aristarchus's Deter- 
mination of the Parallax. — Different Available Methods. — Observa- 
tions of Mars and of the nearer Asteroids. — Transits of Venus. — 
Observations of Contacts and Photographic Work. — Determination 
of Solar Parallax by means of the Velocity of Light ; by Lunar and 
Planetary Perturbations. — Illustrations of the Immensity of the 
Sug's Distance. — Diameter of the Sun. — The Sun's Mass and Density. 

The problem of finding the distance of the sun is 
one of the most important and difficult presented by 
astronomy. Its importance lies in this, that this dis- 
tance — the radius of the earth's orbit — is the base-line 
by means of which we measure every other celestial., 
distance, excepting only that of the moon ; so that error || 
in this base propagates itself in all directions through 
all space, affecting with a corresponding proportion of 
falsehood every measured line — the distance of every 
star, the radius of every orbit, the diameter of every 
planet. 

Our estimates of the masses of the heavenly bodies 
also depend upon a knowledge of the sun's distance 
from the earth. The quantity of matter in a star or 
planet is determined by calculations whose fundamental 
data include the distance between the investigated body 
and some other body whose motion is controlled or 
modified by it ; and this distance generally enters into 
the computation by its cube, so that any error in it in- 



DISTANCE AND DIMENSIONS OF THE SUN. H 

volves a more tlian threefold error in the resulting mass. 
An uncertainty of one per cent, in the sun's distance 
implies an uncertainty of more than three per cent, in 
every celestial mass and every cosmical force. 

Error in this fundamental element propagates itself 
in time also, as well as in space and mass. That is to 
tiay, our calculations of the mutual effects of the planets 
upon each other's motions depend upon an accurate 
knowledge of their masses and distances. By these 
calculations, were our data perfect, we could predict for 
all futurity, or reproduce for any given epoch of the 
past, the configurations of the planets and the con- 
ditions of their orbits, and many interesting problems 
in geology and natural history seem to require for their 
solution just such determinations of the form and po- 
sition of the earth's orbit in by-gone ages. 

]^ow, the slightest inaccuracy in the data, though 
hardly affecting the result for epochs near the present, 
leads to error which accumulates with the lapse of 
time ; so that even the present uncertainty of the sun's 
distance, small as it is, renders precarious all conclu- 
sions from such computations when the period is ex- 
tended more than a few hundred thousand years. If, 
for instance, we should find as the result of calcula- 
tion with the received data, that two millions of years 
ago the eccentricity of the earth's orbit was at a maxi- 
mum, and the perihelion so placed that the sun was 
nearest during the northern winter (a condition of 
affairs which it is thought would produce a glacial 
epoch in the southern hemisphere), it might easily 
happen that our results would be exactly contrary to 
the truth, and that the state of affairs indicated did 
not occur within ten thousand years of the specified 
date — and all because in our calculation the sun's dis- 



12 THE SUN. 

tance, or the solar parallax by wliicli it is measured, 
was assumed half of one per cent, too great or too 
small. In fact, this solar parallax enters into almost 
every kind of astronomical computations, from those 
which deal with stellar systems and the constitution of 
the universe, to those which have for their object noth- 
ing higher than the prediction of the moon's place as 
a means of finding the longitude at sea. 

Of course, it hardly need be said that its determina- 
tion is the first step to any knowledge of the dimensions 
and constitution of the sun itself. 

This " parallax " of the sun is simply the angular 
semi-diameter of the earth as seen from the sun ; or, it 
may be defined in another way as the angle between 
the direction of the sun ideally observed from the center 
of the earth, and its actual direction as seen from a sta- 
tion where it is just rising above the horizon. 

We know with great accuracy the dimensions of the 
earth. Its mean equatorial radius, according to Hark- 
ness's latest determination (agreeing, however, very 
closely with previous ones), is 3963*124 English miles 
[637Y'9Y2 kilometres], and the error can hardly amount 
to more than ^-g^^or ^^ ^^^ whole — perliaps, 800 feet 
one way or the other. Accordingly, if we know how 
large the earth looks from any point, or, to speak tech- 
nically, if we know the parallax of the point, its dis- 
tance can at once be found by a very easy calculation : 
it equals simply [206,265 ^ X the radius of the earth] -v- 
[the parallax in seconds of arc]. 

* This number 206,265 is the length of the radius of a circle ex- 
pressed in seconds of its circumference. A ball one foot in actual diam- 
eter would have an apparent diameter of one second at a distance of 
206,265 feet, or a little more than 39 miles. If its apparent diameter 
were 10", its distance would, of course, be only /y- as great. 



DISTANCE AND DIMENSIONS OF THE SUN. 13 

Now, in the case of the sun it is very difficult to 
find the parallax with sufficient precision on account of 
its smallness — it is less than Q^\ almost certainly between 
8*75'^ and 8*85^'. But this tenth of a second of doubtful- 
ness is more than -^^ of the whole, although it is no 
more than the angle subtended by a single hair at a dis- 
tance of nearly 800 feet. If we call the parallax 8*80'', 
which is probably very near the truth, the distance of 
the sun will come out 92,892,000 miles, while a varia- 
tion of gig- of a second either way will change it about 
half a million of miles. 

When a surveyor has to find the distance of an in- 
accessible object, he lays off a convenient base-line, and 
from its extremities observes the directions of the ob- 
ject, considering himself very unfortunate if he can 
not get a base whose length is at least -^ of the dis- 
tance to be measured. But the whole diameter of the 
earth is less than jy^-q^ of the distance of the sun, 
and the astronomer is in the predicament of a sur- 
veyor who, having to measure the distance of an ob- 
ject ten miles off, finds himself restricted to a base of 
less than five feet ; and herein lies the difficulty of the 
problem. 

Of course, it would be hopeless to attempt this prob- 
lem by direct observations, such as answer perfectly in 
the case of the moon, whose distance is only thirty 
times the earth's diameter. In her case, observations 
taken from stations widely separated in latitude, like 
Berlin and the Cape of Good Hope, or Washington and 
Santiago, determine her parallax and distance with very 
satisfactory precision ; but if observations of the same 
accuracy could be made upon the sun (which is not the 
case, since its heat disturbs the adjustments of an instru- 
ment), they would only show the parallax to be some- 



14 THE SUN. 

where between 8'' and 10'' ; its distance between 126,-1 
000,000 and 82,000,000 miles. 

Astronomers, therefore, have been driven to emploj 
indirect methods based on various principles : some on 
observations of the nearer planets, some on calculations 
founded upon the irregularities — the so-called pertur- 
bations — of lunar and planetary movements, and some 
upon observations of the velocitj^ of light. Indeed, 
before the Christian era, Aristarchus of Samos had de- 
vised a method so ingenious and pretty in theory that 
it really deserved success, and would have attained it 
were the necessary observations susceptible of sufficient 
accuracy. 

His idea was to observe carefully the number of 
hours between new moon and the first quarter, and also 
between the quarter and the full. The first interval 
should be shorter than the second, and the difference 
would determine how many times the distance of the 
sun from the earth exceeds that of the moon, as will 
be clear from the accompanying figure. The moon 

Fig. 1. 





H 


_a 






I 


^^ 


\ 


"~~— — — _ S 


\ 






/N 


45 



reaches its quarter, or appears as a half -moon, when it 
arrives at the point Q, where the lines drawn from it 
to the sun and earth are perpendicular to each other. 
Since the angle H E Q = E S Q, it will follow that 
H Q is the same fraction of H E as Q E is of E S ; so 
that, if H Q can be found, we shall at once have the 
ratio of Q E and E S. Aristarchus thought he had as- 
certained that the first quarter of the month (from N to 



DISTANCE AXD DIMEXSIOXS OF THE SUX. 15 

Q) was about 12 hours shorter than the second, from 
which he computed the sun to be about 19 times as dis- 
tant as the moon. The difficulty lies mainly in the 
impossibility of determining the instant when the disk 
of the moon is exactly bisected, and depends partly 
upon the fact that the lunar surface is very rough, and 
partly upon the fact that the sun's diameter is nearly 
twice that of the orbit of the moon, instead of being a 
mere point, as in the figure. The boundary between 
light and darkness — the terininator^ as it is called — is 
both irregular and ill-defined. The real difference be- 
tween the two quarters is not quite 36 minutes, so that 
the sun's distance is about 400 times the moon's. For 
more than 1,500 years, however, the result of Aristar- 
chus stood unquestioned, having been accepted by Hip- 
parchus and Ptolemy. 

The different methods upon which our present 
knowledge of the sun's distance depends may be classi- 
fied as follows : 

1. Observations upon the planet Mars near opposition, in two dis- 

tinct wavs : 
(a) Observations of the planet's declination made from sta- 
tions widelj separated in latitude. 

(5) Observations from a single station of the planet's right 

ascension when near the eastern and western horizons 
— known as Flamsteed's or Bond's method. 

2. Observations of Venus at or near inferior conjunctions: 

(a) Observations of her distance from small stars measured 
at stations widely different in latitude. 

(6) Observations of the transits of the planet : 1. By noting 

the duration of the transit at widely-separated sta- 
tions ; 2. By noting the true Greenwich time of con- 
tact of the planet with the sun's limb ; 3. By measur- 
ing the distance of the planet from the sun's limb with 
suitable micrometric apparatus ; 4. By photographing 
the transit, and subsequently measuring the pictures. 



16 THE SUN. 

3. By observiDg the oppositions of the nearer asteroids in the same 

manner as those of Mars. 

4. By means of the so-called parallactic inequality of the moon. 

5. By means of the monthly equation of the sun's motion. 

6. By means of the perturbations of the planets, which furnish us . 

the means of computing the ratios between the masses of 1 
the planets and the sun, and consequently their distances — j 
known as Leverrier's method. 

7. By measuring the velocity of light, and combining the result j 

{a) with *^ equation of light" between the earth and sun, or 
(b) with " the constant of aberration." 

Our scope and limits do not, of course, require or 
allow any exhaustive discussion of these different meth- 
ods and their results, but some of them will repay a few 
moments' consideration : 

The first three methods, known as the trigono- 
metrical methods^ are all based upon the same general 
idea, that of finding the actual distance of one of the 
nearer planets by observing its displacement in the sky 
as seen from remote points on the earth. The rela- 
tive distances of the planets are easily found in sev- 
eral diflferent ways,^ and are known w^ith very great 

* One method of determining the relative distances of a planet and 
the sun from each other and from the earth is the following, known since 
the days of Eipparchus ; First, observe the date when the planet comes 

Fig. 2. 



II 




to its opposition — i. e., when sun, earth, and planet are in line, as in the 
figure, where the planet and earth are represented by M and E. Next, 
after a known number of days, say one hundred, when the planet has ad- 



DISTANCE AND DIMENSIONS OF THE SUN. 



17 



accuracy — the possible error hardly reaching the ten- 
thousandth in even the most unfavorable cases. In 
other words, we are able to draw for any moment an 
exceedingly accurate map of the solar system — the only 
question being as to the scale. Of course, the determi- 

Fig. 3. 




nation of any line in the map will fix this scale ; and 
for this purpose one line is as good as another, so that 
the measurement of the distance from the earth to the 
planet Mars, for instance, will settle all the dimensions 
of the system. 

vanced to M' and the earth to E', observe the planet's elonjration frona 
the sun, i. e., the' angle M' E' S. Now, since we know the periodic times 
of both the earth and planet, we shall know both the angle M S M' moved 
over by the planet in one hundred days, and also E S E', described in the 
same time by the earth. The difference is M' S E, often called the synodic 
angle. We have, therefore, in the triangle M'SE',the angle at E' 
measured, and the angle M' S E' known as stated above, and hence by 
the ordinary processes of trigonometry we can find the relative values 
of its three sides, 
3 



i 



18 THE SUN. 

Fig. 3 illustrates the method of observation. Sup- 
pose two observers, situated one near the north pole 
of the earth, the other near the south. Looking at the 
planet, the northern observer will see it at N (in the upper 
figure), while the other will see it at S, farther north in 
the sky. If the northern observer sees it as at A (i 
the lower part of the figure), the southern will at th 
same time see it as at B ; and, by measuring carefully! 
at each station the apparent distance of the planet from 
several of the little stars {a^ ?>, c) which appear in the 
field of view, the amount of the displacement can be 
accurately ascertained. The figure is drawn to scale. 
The circle E being taken to represent the size of the 
earth as seen from Mars when nearest us, the black disk 
represents the apparent size of the planet on the same 
scale, and the distance between the points N and S, in 
either figure A or B, represents, on the same scale also, 
the displacement which would be produced in the plan- 
et's position by a transference of the observer from 
Washington to Santiago, or vice versa. 

The first modern attempt to determine the sun's 
parallax was made by this method in 1670, when the 
French Academy of Sciences sent Richer to Cayenne to 
observe the opposition of Mars, while Cassini (who pro- 
posed the expedition), Koemer, and Picard observed it 
from different stations in France. When the results 
came to be compared, however, it was found that the 
planet's displacement was imperceptible by their existJ I 
ing means of observation : from this they inferred that 
the planet's parallax could not exceed half a minute of 
arc, and that the sun's could not be more than 10''. 

In 1752 Lacaille at the Cape of Good Hope made 
similar observations, and their comparison with cor- 
responding observations in Europe showed that instru- 



DISTANCE AND DIMENSIONS OF THE SUN. 19 

merits had so far improved as to make the displacement 
quite sensible. He fixed the sun's parallax at 10'^, cor- 
responding to a distance of 82,000,000 miles. 

In more recent times the method has been frequently 
applied. It can be used to the best advantage, of course, 
when, at the time of " opposition," the planet is near its 
perihelion and the earth near its aphelion, for then the 
distance between Mars and the earth is the least possi- 
ble. These favorable oppositions occur in the late sum- 
mer or early autumn about once in fifteen years, as in 
1847, 1862, 1877, and 1892. 

The meridian observations which furnish the ma- 
terial of method la^ and were mainly relied on until re- 
cently, seem for some reason, perhaps connected with 
the planet's red color, to be untrustworthy ; at any rate, 
they generally give values of the parallax persistently 
about one per cent, larger than the other methods, and 
rather discordant among themselves. 

riamsteed's method, on the other hand, stands very 
high, especially when so modified as to utilize the co- 
! operation of numerous observers in different countries. 
Though first suggested long ago, it would have amounted 
to very little with the instruments then available, and it 
had been practically lost sight of until the expedition of 
' Dr. Gill to Ascension Island, in 1877, brought out its 
real value. 

] His instrument was a " heliometer," loaned by Lord 
\ Lindsay for the occasion. It consists essentially of a 
I telescope having its object-glass divided into two semi- 
I circular pieces which can slide by each other. Each 
' half of the lens makes its own image of the object under 
i examination, so that by properly setting the semi-lenses 
' the images of two neighboring stars can be made to 
j coincide ; and if we know the displacement of the two 



20 THE SUN. 

lenses, wliich can be measured by an accurate scale, the 
angular distance between the stars can be determined 
with a precision unattainable by any other known pro> 
cess. The instrument is delicate, complicated, and diffi- 
cult to use, but in the hands of an adept it is thoroughly 
reliable. It was with the heliometer that Bessel, in 1838, 
first sounded interstellar space by measuring the annual 
parallax^ and distance of 61 Cygni. 

Mr. GilPs observations consisted in measurements 
of the apparent distance between the planet and the 
stars lying near its path, and of the distances between 
the stars themselves ; the principal observatories also 
co-operated in the work by determining wdth the utmost 
precision the absolute places of the stars. It would 
take too much space to explain fully how from such 
observations the solar parallax can be accurately worked 
out ; but any one can easily see that when the planet is 
rising the efl^ect of parallax (which always makes a body 
appear lower in the heavens than it otherwise would) is to 
shift it apparently toward the east ; when Mars is in the 
west the apparent shift, on the other hand, is w^estward • 
and by comparing the measurements made at all houn 
of the night for several consecutive weeks the planet's 
regular orbital motion and the amount of this daily 
parallactic shift can be separately determined with mi- 
nute exactness. 

As the final result of the whole operation. Dr. Gill 
obtained 8-780'' ± 0*020'' for the sun's parallax. 

Several of the minor planets or asteroids which have 
very eccentric orbits at times come so near us at oppo- 

* The '' annual," or " heliocentric," " parallax " of a star is not the 
same as its horizontal parallax, or angular semi-diameter of the earth as 
seen from the star ; it is the semi-diameter of the earth's orhit viewed i 
from the star, and is nearly twelve thousand times greater than the other. 



DISTANCE AND DIMENSIONS OF THE SUN. 21 

sition that tliey can be advantageously observed in the 
same way. They never approach quite as close as Mars 
does, but per contra they are so much smaller that 
they look just like stars, and can be observed with the 
heliometer much more accurately than a planet whicli 
presents a disk. Very recently, in 1889 and 1890, a 
concerted system of observations was made upon Vic- 
toria, Iris, and Sappho by Dr. Gill, now the Astronomer 
Eoyal at the Cape of Good Hope ; Dr. Elkin, of the 
Yale College Observatory (which possesses a fine heli- 
ometer, an exact mate of Dr. Gill's, and the only one 
in the United States), and two or three German ob- 
servers with smaller instruments. The results are very 
satisfactory, ranging from 8*796^^ to 8*825'', the mean 
being 8-807'', with a probable error of only 0-006". 

So far as can be judged from the details thus far 
published, this determination must be conceded the pre- 
cedence over all others in respect to its probable freedom 
from constant and systematic errors, and from theoret- 
ical difiiculties. 

In observations of this sort upon Mars or the aster- 
oids, the position and displacement of the planet, as 
seen from different stations, are determined by com- 
paring it with neighboring stars. When Venus, how- 
ever, is nearest us, she can be observed onW by day, so 
that in her case star comparisons are as a general thing 
out of the question. But occasionally at her inferior 
conjunction she passes directly across the disk of the 
sun, the phenomenon being known as a ^'transit." 
These transits are very rare, coming (at present) in 
pairs, the two transits which constitute a pair being 
separated by an interval of eight years, while between 
the pairs themselves there is an interval of either one 
hundred and thirty or one hundred and thirteen years. 



22 THE SUN. 

They occur either in Jane or December, and thus far 
there have been six since the invention of the telescope ; 
viz., in December, 1631 and 1639 ; in June, 1761 and 
1769 ; and in December, 1874 and 1882, the next pair 
being due in June, 2004 and 2012. 

On these occasions the parallactic displacement of 
the planet as seen from diiferent stations can be deter- 
mined by making any such observations as will enable 
the computer to ascertain accurately her apparent dis- 
tance and direction from the sun's center at some given 
moment. 

Gregory in 1663 first pointed out the utility of such 
observations for ascertaining the parallax, but it was 
not until some fifteen years later that the subject was 
fairly brought to the attention of astronomers by Hal- 
ley, who discussed the matter thoroughly, and showed 
how the problem might be solved with accuracy by 
observations such as were practicable even with the 
instruments and knowledge then at command. 

From that time for fully two hundred years it was 
the almost universal opinion of astronomers that no 
other method could rival this as a means of determining 
the distance of the sun. 

The transits of 1761 and 1769 were observed in all 
accessible quarters of the globe by expeditions sent out 
by the diflierent governments. From different sets of 
these observations variously combined by different com- 
puters, values of the solar parallax were obtained ranging 
all the way from 7*5'' to 9*2''. A general discussion of 
all the material afforded by the two transits was first 
made by Encke in 1822, and he obtained, as the most 
probable result, the value 8*5776'', which from that time 
for more than thirty years was accepted by all astrono- 
mers as the best attainable approximation to the truth. 



II 



DISTANCE AND DIMENSIONS OF THE SUN. 



23 



In 1854 Hansen, in publishing some of his results 
respecting the motion of the moon, announced that 
Encke's value of the solar parallax could not be recon- 
ciled with his investigations ; within the next six or seven 
years several independent researches by other astrono- 
mers confirmed his conclusions ; and the more recent 
recomputations by Powalkj^, Stone, Faye, and others, 
show that the errors of observation were so considerable 
in 1769 that nothing more can be fairly deduced from 
that transit than that the solar parallax is probably 
somewhere between 8*7'^ and 8'9'^ 

The method of observation then used consisted sim- 
ply in noting the moment when the limb of the planet 
came in contact with that of the sun — an observation 
which is attended with much more difficulty and uncer- 
tainty than would at first be supposed. The difficul- 
ties depend in part upon the imperfections of optical 
instruments and the human eye, partly upon the essen- 

FiG. 4. 




tial nature of light, leading to what is known as diffrac- 
tion, and partly upon the action of the planet's atmos- 
phere. The two first-named causes produce what is 
called irradiation, and operate to make the apparent 
diameter of the planet, as seen on the solar disk, small- 



24 THE SUN. 

er than it really is — smaller, too, by an amount which 
varies with the size of the telescope, the perfection of 
its lenses, and the tint and brightness of the sun's image. 
The edge of the planet's image is also rendered slightly 
hazy and indistinct. 

The planet's atmosphere also causes its disk to be 
surrounded by a narrow ring of light, which becomes 
visible long before the planet touches the sun, and at 
the moment of internal contact produces an apyjearance 
of which the accompanying figure is intended to give 
an idea, though on an exaggerated scale. The planet 
moves so slowly as to occupy more than twenty minutes 
in crossing the sun's limb ; so that, even if the planet's 
edge were perfectly sharp and definite, and the sun's 
limb undistorted, it would be very difficult to deter- 
mine the precise second at which contact occurs ; but as 
things are, observers with precisely similar telescopes, 
and side by side, often differ from each other five or 
six seconds ; and where the telescopes are not similar 
the differences and uncertainties are much greater. 
The extent of the difficulty can be judged of by the 
simple fact that, from the whole mass of contact obser- 
vations obtained in 1874 by the different British parties 
which observed the transit, three different values of the 
solar parallax have been deduced by diff'erent computers, 
viz., the official value 8*76'' by Airy, 8*81'' by Tupman, 
and 8*88'' by Stone. These differences depend mainly 
upon the different interpretations given to the de- 
scription of phenomena noted by the observers in the 
field. 

In 1882 things were perhaps a little better, as many 
of the observers had the benefit of experience in 1874. 
Professor Newcomb deduces from all the observations 
of internal contact in the two transits a solar parallax 



DISTANCE AND DIMENSIONS OF THE SUN. 25 

of 8-776'' ± 0-023''. But many of the several hundred 

j observations were seriously discordant. 

The difficulties of the case were fully realized at the 
time when preparations were making for the observa- 
tion of the transit of 1874, and astronomers were dis- 
posed to put more reliance upon micrometric and pho- 
tographic methods, which are free from these peculiar 
difficulties, though of course beset with others, which, 

; however, it was hoped would prove less formidable. 

All the numerous expeditions, therefore, w^hich were 
sent out by the various governments to observe the 
transits of 1874 and 1882 were equipped to use one or 
both these methods. 

All the eight German parties, two or three of the 

j Russian parties, one English, and one Belgian party 
were provided with heliometers, and busied themselves 
during the transit with measuring the distance of the 

1 planet from the edge of the sun's disk. The results of 
the German observations have been fully worked out 
and published. From the four hundred and forty-six 
different measures Auwers deduces a solar parallax of 
8*878" ± 0-040" ; the value is surprisingly large, but the 
magnitude of its probable error indicates that the obser- 
vations did not agree very closely. 

The Americans and French placed their main reli- 
ance upon the photographic method, while the English 
and Germans also provided for its use to a certain 
extent. The great advantage of this method is that 
it makes it possible to perform the necessary measure- 
ments, upon whose accuracy everything depends, at 
leisure after the transit, without hurry, and with all 
possible precautions. The field-work consists merely in 
obtaining as many and as good pictures as possible. A 
principal objection to the method lies in the difficulty 



26 THE SUN. 

of obtaining good pictures — i. e., pictures free from dis^ 
tortion, and so distinct and sharp as to bear high mag- 
nifying power in the microscopic apparatus used for 
their measurement. A most, serious difficulty, more-| 
over, is involved in the accurate determination of the 
scale of the picture — that is, of the number of seconds 
of arc corresponding to a linear inch upon the plate. 
Besides this, we must know the exact Greenwich time 
at which each picture is taken, and it is also extremely 
desirable that the orientation of the picture should be 
accurately determined — that is, the north and south, east 
and west points of the solar image on the finished plate. 
There has been a good deal of anxiety lest the image, 
however accurate and sharp when first produced, should 
alter in course of time through the contraction of the 
collodion or gelatine film on the glass plate, but the ex- 
periments of Rutherfurd, Huggins, and Paschen seem 
to show that this danger is imaginary. 

The uncertainty of our present knowledge of the 
sun's parallax is, however, so small that we can hope to 
improve it only by means of photographs that are 
almost absolutely perfect. Unless the picture is so dis- 
tinct and free from distortion that the relative positions 
of Yenus and the sun's center can be determined from 
it on the four-inch disk within -g-^-o of an inch the plate 
is practically worthless. 

But it is to be noted that any mere enlargement or 
diminution of the diameter of sun or planet will do no 
harm, provided it is alike all around the circumference 
of the disk, since the measurement is not from the edge 
of Yenus to the edge of the sun, but between their cen- 
ters. Photographic determinations of contact^ on the 
contrary (such as Janssen and some of the English par- 
ties attempted by a peculiar and complicated apparatus). 



DISTANCE AND DIMENSIONS OF THE SUN. 27 

are affected with all the uncertaiiities of the old-fash- 
ioned observations of the eye alone, and with others in 
addition ; so that, astronomically considered, they are 
entirely worthless, although interesting from a chemical 

i and physical point of view. 

In 1874 two essentially different lines of proceeding 
were adopted in the photographic observations. The 
English and Germans attached a camera to the eye- 
end of an ordinary telescope; which was pointed directly 
at the sun ; the image formed at the focus of the tele- 
scope was enlarged to the proper size by a combination 
of lenses in the camera ; and a small plate of glass ruled 
with squares was placed at the focus of the telescope 
and photographed with the sun's image, furnishing a 
set of reference-lines, which give the means of detecting 

i and allowing for any distortion caused by the enlarging 
lenses. 

The Americans and French, on the other hand, pre- 
ferred to make the picture of full size, without the in- 
tervention of any enlarging lens : as this requires an 
object-glass with a focal length of thirty or forty feet, 
which could not be easily pointed at the sun, a plan 
proposed first by M. Laussedat, but also independently 
by our own Professor Winlock, was adopted. The tele- 
scope is placed horizontal, and the rays are reflected into 
the object-glass by a plane mirror suitably mounted. 
The French used mirrors of silvered glass, and took 
their pictures (about two and a half inches in diame- 
ter) by the old daguerreotype process on silvered plates 
of copper, in order to avoid the risk of collodion-con- 
traction. With the silvered mirror the time of expo- 
sure is so short that no clock-w^ork is required. The 
Americans used unsilvered mirrors, to obviate any dis- 
torting action of the sun's rays upon the form of the 



28 



THE SUN. 



mirror. This, of course, made the light feebler and 
the time of exposure longer, so that a clock-work move- 
ment of the mirror was needed to keep the image from 



Fig. 5. 





American Apparatus for Photographing the Transit of Venus. 

changing its place on the plate during tlie exposure, 
which, however, never exceeded half a second. Fig. 
5, taken from the author's "General Astronomy" by 
permission of the publishers, gives an idea of the ar- 
rangement. The pier that carries the plate was in a 
darkened room, into which the rays from the mirror 
were admitted by a sliding shutter. 

In 1874 the American pictures were taken by the 
ordinary wet process on glass, and were about four 
inches in diameter. In 1882 a gelatine emulsion pro- 
cess was used. Just in front of the sensitive plate, at a 
distance of about one eighth of an inch, was placed a 
reticle, or a plate of glass ruled in squares, and between 
this and the collodion-plate hung a line silver ware sus- 
pending a plumb-bob. Thus the finished negative w^as 



DISTANCE AND DIMENSIONS OF THE SUN. 



29 



marked into squares, and also bore the image of the 
plumb-line, which indicated precisely the direction of 
the vertical. The Americans also placed the photo- 
graphic telescope exactly in line with a meridian instru- 
ment, and so determined, with the extremest precision, 
the direction in which it was pointed. Knowing this, 
and the time at which any picture was taken, it becomes 
possible, Avith the help of the plumb-line image, to de- 
termine precisely the orientation of the j^icture — an ad- 

FiQ. 6. 







9 






m 






■ 






• ^ 








IH 


■ 
















^ 


■ 


■ 










^ 




,"" 




1 


B 












CHIN 


A 




1 


1 




^L^ 


^- 










. 


1 


I 














Am 


1 






mi 






■ 








H 



vantage possessed by the American pictures alone, and 
making their value nearly twice as great as otherwise it 
would have been. 

The above figure is a representation of one of the 
American photographs reduced about one half. Fis 
the image of Venus, which on the actual plate is about 
one seventh of an inch in diameter ; a a' \'^ the image 
of the plumb-line. The center of the reticle is marked 



30 THE SUN. 

by the little cross, and the word " China," written on 
the reticle-plate with a diamond — and, of course, copied 
on the photograph — indicates that it is one of the Peking 
pictures. Its number in the series is given in the right- 
hand upper corner. About 90 such pictures were ob- 
tained at Peking during the transit, and about 350 at 
all the eight American stations, the work being much 
interfered with by unfavorable weather at most of them. 
If we add those obtained by the French, Germans, and 
English, the total number available reaches nearly 1,200, 
according to the best estimates. 

After the pictures are made and safely brought 
home, they have next to be measured — i. e., the dis- 
tance (and in the American pictures the direction also) 
between the center of Venus and the center of the sun 
must be determined in each picture. This is an exceed- 
ingly delicate and tedious operation, rendered more dif- 
ficult by the fact that the image of the sun is never 
truly circular, but, even supposing the instrument to be 
perfect in all its adjustments, is somewhat distorted by 
the effect of atmospheric refraction; so that the trueil 
position of the sun's center with reference to the squares "' 
of the reticle is determined only by an intricate calcula- 
tion from measurements made with a microscopic ap- 
paratus on a great number of points suitably chosen on 
the circumference of the image. The final result of the 
measurement comes out something in this form : Peking. 
No. 32. Time, 14^ 08"^ 20-2' (Greenwich mean time) ; 
Yenus north of sun's center, Y35*32'' ; east of center, 
441-63'' ; distance from center of sun, 857*75^ (The 
numbers given are only imaginary.) 

In 1882 less prominence was given to photographic ■ 
operations by most of the Government expeditions, since 
the results of the work in 1874, so far as then published, 



^1 



ti 



I 



DISTANCE AND DIMENSIONS OF THE SUN. 31 



were not very satisfactory. The American parties, 
however, adhered to the same apparatus and methods as 
in 1874, except that tlie collodion process was replaced 
by an emulsion. Nearly 1,500 photographs were ob- 
tained. From the whole system of American photo- 
graphs Professor Newcomb deduces a solar parallax of 
8*857'^ ± O'OIG^''. The measures of distance alone give 
8*867^^, but those oi position-angles g^\Q 8-873^'' in 1874, 
and 8-772'' in 1882. 

The discordances between the results from different 
plates, made within a few minutes of each other, show 
that there is something wrong with the method. The 
most probable explanation is perhaps to be found in the 
distortions suffered by the plane mirror of the apparatus 

I under changes of position and temperature. 

From the 92 French daguerreotypes of 1874 a paral- 
lax of 8-80'' ± 0-03'' was deduced by Obrecht. 

\ The English photographs of 1874 proved of little 

I value. They were measured by two different persons, 
and from the measurements of one (Mr. Burton) a par- 

i allax of 8*25'' was deduced, while from those of the 
other (Captain Tupnian) the result was 8*08''. One of 

! the principal difficulties evidently lay in the uncertainty 
of the scale-value, which was only deduced from the di- 
ameters of the sun and planet. 

On the whole, it may be taken as certain that here- 
after transits of Venus will not be considered of such 
supreme importance as in the past. Other less costly 
operations will give better results for the solar parallax. 
The methods numbered 4, 5, and 6, on page 16, 
are usually classed together as " gravitational^^'^ since 
they depend on calculations which are founded on the 
law of gravitation. One of the best of them is based 
upon the careful observation of the motions of the 



32 THE SUN. 

moon. The first suspicion as to the correctness of the 
then received distance of the sun was raised in 1854 by 
Hansen's announcement that the moon's parallactic in- 
equality led to a smaller value than that deduced from 
the transit of Venus — a conclusion corroborated by 
Leverrier four years later, from the so-called lunar 
equation of the sun's motion. It seems at first sight 
strange, but it is true, as Laplace long since pointed out, 
that the skillful astronomer, by merely watching the 
movements of our satellite, and without leaving his ob- 
servatory, can obtain the solution of problems which, 
attacked by other methods, require tedious and expen- 
sive expeditions to remote corners of the earth. Our 
scope and object do not require us to enter into detail 
respecting this lunar method of finding the sun's paral- 
lax ; it must suffice to say that the disturbing action of 
the sun makes the interval from new moon to the first 
quarter about eight minutes longer than that from the 
quarter to full ; and this difference depends upon the 
ratio hetween the diameter of the moon^s orbit and the 
distance of the sun in such a manner that, if the in- 
equality is accurately observed, the ratio can be cal- 
culated. Since we know the distance of the moon, this 
will give that of the sun. The results obtained in this 
way, according to the most recent investigations, ap- 
pear to fix the solar parallax between 8*76Y''' and 8*802''. 
Newcomb assigns 8*794''' as the weighted mean. 

But the method by which ultimately we shall obtain 
the most accurate determination of the dimensions of 
our system is that proposed by Leverrier, depending 
upon the secular perturbations produced by the earth 
upon her neighboring planets ; especially in causing the 
motions of their nodes and perihelia. These motions 
are very slow, but continuous / and hence, as time goes, 






DISTANCE AND DIMENSIONS OF THE SUN. 33 

J, on, they will become known witli ever-increasing accu- 
racy. If they were known with absolute precision^ they 
would enahle us to compute^ with absolute precision 
J also^ the ratio between the masses of the sun and earthy 
r. and from this ratio w^e can calculate * the distance of 
^the sun by either of two or three different methods. 

As matters stand at present, the majority of astrono- 
mers would probably consider that these secular pertur- 
c bations are not yet known with an exactness sufficient 
to render this method superior to the others that have 
been named — perhaps as yet not even their rival. Le- 
. verrier, on the other hand, himself put such confidence 
. in it that he declined to sanction or co-operate in the 
J operations for observing the recent transit of Venus, 
J considering all labor and expense in that direction as 
I merely so much waste. 

I But, however the case may be now, there is no 
j question that as time goes on, and our knowledge of 
1 the planetary motions becomes more minutely precise, 
I this method will become continually and cumulatively 
! more exact, until finally, and not many centuries hence, 
it will supersede all the others that have been described. 

j * One method of proceeding is as follows : Let M be the mass of the 
j sun and earth united, and m that of the earth and moon ; let R be the 
distance of the sun from the earth, and r that of the moon ; finally, let 
I T be the number of days in a sidereal year, and t the number in a side- 
real month. Then, by elementary astronomy — 

M:^^^:^^ whence R3:=.3MYm 
T'^ t^ \t^ J \m 

or, in words, the cube of the sun^s distance equals the cube of the moon^s 
distance^ multiplied by the square of the number of sidereal months in a 
year^ and by the ratio between the masses of the sun and earth. It is to be 
noted, however, that T and t are the periods of the earth and moon, as 
they would be if wholly undisturbed in their motions, and hence differ 
slightly from the periods actually observed — the differences are small, 
but somewhat troublesome to calculate with precision. 

4 



34 THE SUK 

The parallax of the sun determined by Leverrier in 
this method, in 1872, came out 8*86^^ 

Professor Newcomb, as the result of his recent ex- 
haustive researches upon the subject, gets 8*Y59^' ± , 
0-010^ 

The last of the methods mentioned in the synopsis 
given on pages 15 and 16 is interesting as an example of 
the manner in which the sciences are mutually connected 
and dependent. Before the experiments of Fizeau in 
1849, and of Foucault a few years later, our knowledge 
of the velocity of light depended on our knowledge of 
the dimensions of the earth's orbit. It had been found 
by astronomical observations upon the eclipses of Jupi- 
ter's satellites that light occupied a little more than six- 
teen minutes in crossing the orbit of the earth, or about 
eight minutes in coming from the sun ; and hence, 
supposing the sun's distance to be 95,600,000 miles, as ; 
was long believed, the velocity of light must be about i 
192,000 miles per second. Thus optics was indebted to 
astronomy for this fundamental element. But when 
Foucault in 1862 announced that, according to his un- \ 
questionably accurate experiments, the velocity of light 
could not be nmch more than 186,000 miles per second, 
the obligation was returned, and the suspicions as to I 
the received value of the sun's parallax, which had been | 
raised by the lunar researches of Hansen and Leverrier, 
were changed into certainty. 

The most accurate determinations of the velocity of 
light have been made in this country by Michelson and 
Newcomb, between 18Y9 and 1883, and give as the re- 
sult 186,327 miles, with a probable error not exceeding 
twenty miles. 

From this we can derive the distance of the sun di- 
rectly by merely multiplying it by the " constant of the 



DISTANCE AXD DIMENSIONS OF THE SUN. 35 

equation ofligM^'^ which is simply the number of seconds 
required by light to travel from the sun to the earth. 
This "constant" is determined by observation upon the 
eclipses of Jupiter's satellites, and is almost certainly 
very near 499 seconds, though still doubtful by a frac- 
-tion of a second. This would give 92,977,000 miles for 
the sun's distance, corresponding to a parallax of about 
8*79'^ During the last twelve or fifteen years continu- 
Fous series of observations have been in progress by new 
photometric methods both at Cambridge (U. S.) and 
^ Paris, and when their result is published we shall un- 
doubtedly have a much more accurate value of the light- 
equation. 

The velocity of light may be utilized in another way 
to solve the problem, by combining it with the so-called 
' " constant of aberrations^ This " aberration constant " 
^ is deduced from observations upon the fixed stars, and 
^'almost certainly lies somewhere between 20*45'^ and 
^j 20-55'^, corresponding to parallaxes of 8*81'^ and 8*77^ 
^ Its determination, however, is somewhat embarrassed by 
■ the newly discovered " variation of latitude," and it is 
' expected that new determinations, in which this varia- 
tion is duly eliminated or taken into account, will give 
a much more accurate value of the aberration. 
; The only difficulty with these two methods lies in 
, the theoretical question whether we can safely assume 
that in interplanetary space the velocity of light is 
identical with that determined by experiments made at 
the surface of the earth, even after all known corrections 
for the density of the air, etc., have been applied. 

Admitting it, there can hardly be a doubt that this 
'^ physical method^^ as it is often called, outranks all 
others for the present as a means of determining the 
distance of the sun ; and the reader's attention is called 



36 THE SUN. 

to the fact that it gives directly the distance of the sun, 
and the parallax only indirectly. It does not depend 
at all upon our measures of the dimensions or gravitai- 
tional attraction of the earth. || 

Collecting all the evidence at present attainable, it 
would seem that the solar parallax can not differ much 
from 8-80'^, though it may be as much as 0*01'^ greater or 
smaller ; this would correspond, as has already been said, 
to a distance of 92,892,000 miles, with a probable error 
of about one eighth of one per cent., or 120,000 miles."^ 

But, though the distance can easily be stated in fig- 
ures, it is not possible to give any real idea of a space 
so enormous ; it is quite beyond our power of concep- 
tion. If one were to try to walk such a distance, sup- 
posing that he could walk 4 miles an hour, and keep it 
up for 10 hours every day, it would take 68|- years to 

* The oscillations of scientific opinion as to the value of this constant 
have been very curious. Early in the century Laplace, in the " Mecanique 
Celeste," adopted the value 8*81" given by the first discussion of the tran- 
sits of Yenus in 1761-'69 ; but other astronomers, Delambre, for instance, 
proposed a smaller value. Encke, as has been said before, made a new 
and thorough discussion of these transits in 1822-'24, and deduced the 
value 8'58", which held the ground for nearly forty years. About 1860 
the researches of Hansen, Leverrier, and Stone were thought to have 
established a value exceeding 8'90", and the "British Nautical Almanac " 
used 8*95" until the issue for 1882. In 1867 Newcomb published a care- 
ful investigation, based upon all the data then known, and deduced the 
value 8*848". Leverrier, in 1872, found 8*86" from the planetary perj 
turbations. The "American Ephemeris," "British Nautical Almanac," 
and the Berlin " Jahrbuch " use Newcomb's value, and the French " Con- 
naissance de Temps" employs Leverrier's. It appears, however, per* 
fectly certain, from the work of the last few years, that the figures (8*80") 
given in the text are much nearer to the truth. Newcomb, in his " Astro- 
nomical Constants" (January, 1895) gives, as the final value based upon 
all available data, 8*797" ± 0004. Harkness, in his " Solar Parallax and 
its related Constants," deduces as the result of a most exhaustive dis- 
cussion 8-809" ± 0-006. 



DISTANCE AND DIMENSIONS OF THE SUN. S7 

. make a single million of miles, and more than 6,300 

i years to traverse the whole. 

If some celestial railway could be imagined, the 
journey to the sun, even if our trains ran 60 miles an 

! hour, day and night and without a stop, would require 

. over 175 years. Sensation, even, would not travel so 
far in a human Kfetime. To borrow the curious illus- 
tration of Professor Mendenhall, if we could imagine 

, an infant with an arm long enough to enable him to 
touch the sun and burn himself, he would die of old 
age before the pain could reach him, since, according 
to the experiments of Helmholtz and others, a nervous 
shock is communicated only at the rate of about 100 
feet per second, or 1,637 miles a day, and would need 
more than 150 years to make the journey. Sound 
would do it in about 14 years if it could be transmitted 
through celestial space, and a cannon-ball in about 9, if 
it were to move uniformly with the same speed as when 
it left the muzzle of the gun. If the earth could be 
suddenly stopped in her orbit, and allowed to fall unob- 
structed toward the sun under the accelerating influence 
of his attraction, she would reach the center in about 
two months. I have said if she could be stopped, but 
such is the compass of her orbit that, to make its circuit 
in a year, she has to move nearly 19 miles a second, or 
more than fifty times faster than the swiftest rifle-ball ; 
and in moving 20 miles her path deviates from perfect 
straightness by less than one eighth of an inch. And 
yet, over all the circumference of this tremendous orbit, 
the sun exercises his dominion, and every pulsation of 
his surface receives its response from the subject earth. 
By observing the slight changes in the sun's ap- 
parent diameter, we find that its distance varies some- 
what at different times of the year, about 3,000,000 



38 THE SUN. 

miles in all ; and minute investigation shows tliat the 
earth's orbit is almost an exact ellipse, whose nearest 
point to the sun, or perihelion^ is passed by the earth 
about the 1st of January, at which time she is 91,385,000 
miles distant. 

The distance of the sun being once known, its di- 
mensions are easily ascertained — at least, within certain 
narrow limits of accuracy- The angular semi-diameter 
of the sun when at the mean distance is almost exactly 
962'', the uncertainty not exceeding ^-^^ of the whole. 
The result of twelve years' observations at Greenwich 
(1836 to 1847) gives 961*82'', and other determinations 
oscillate around the value first mentioned, which is that 
adopted in the " American Nautical Almanac." Taking 
the distance as 92,885,000 miles, this makes the sun's 
diameter 866,400 ; and the probable error of this quan- 
tity, depending as it does hoth on the error of the meas- 
ured diameter and of the distance, is some 4,000 or 
5,000 miles; in other words, the chances are strong that 
the actual diameter is between 860,000 and 870,000 
miles. 

Measurements made by the same person, however, 
and with the same instrument, but at different times, 
sometimes differ enough to raise a suspicion that the 
diameter is slightly variable, which w^ould be nothing 
surprising considering the nature of the solar sur- 
face. 

There is no sensible difference between the equa- 
torial and polar diameters, the rotation of the sun on its 
axis not being sufficiently rapid to make the polar com- 
pression (which must, of course, necessarily result from 
the rotation) marked enough to be perceived by our 
present means of observation. 

It is not easy to obtain any real conception of the 



DISTANCE AND DIMENSIONS OF THE SUN. 39 

vastness of this enormous sphere. Its diameter is 109-5 
times that of the earth, and its circumference propor- 
tional ; so that the traveler who could make the circuit 
of the world in 80 days would need nearly 24 years 
for his journey around the sun. Since the surfaces of 
spheres vary as the squares, and bulks as the cubes, of 
their diameters, it follows that the sun's surface is near- 
ly 12,000 times, and its volume, or bulk, more than 
1,300,000 times, greater than that of the earth. If the 
earth be represented by one of the little three-inch 
globes common in school apparatus, the sun on the same 
scale will be more than 27 feet in diameter, and its dis- 
tance nearly 3,000 feet. Imagine the sun to be hol- 
lowed out and the earth placed in the center of the 
shell thus formed, it would be like a sky to us, and the 
moon would have scope for all her motions far within 
the inclosing surface ; indeed, since she is only 240,000 
miles away, while the sun's radius is more than 430,000, 
there would be room for a second satellite 190,000 miles 
beyond her. 

The mass of the sun, or quantity of matter con- 
tained in it, can also be computed when we know its 
distance, and comes out nearly 330,000 times as great 
as the earth. The calculation may be made either by 
means of the proportion given in the note to page 33, or 
by comparing the attracting force of the sun upon the 
earth, as indicated by the curvature of her orbit (about 
0-119 inch per second), with the distance a body at the 
surface of the earth falls in the same time under the 
action of gravity, a quantity which has been determined 
with great accuracy by experiments with the pendulum. 
Of course, the fact that the sun produces its effect upon 
the earth at a distance of 93,000,000 miles, while a fall- 
ing body at the level of the sea is only about 4,000 



40 THE SUN. 

miles from the center of the attraction which produces 
its motion, must also enter into the reckoning.^ 

This mass, if we express it in pounds or tons, io too 
enormous to be conceived : it is 2 octillions of tons — 
that is, 2 with 27 ciphers annexed ; it is nearly 750 
times as great as the combined masses of all the planets 
and satellites of the solar system — and Jupiter alone is 
more than 300 times as massive as the earth. The sun's 
attractive power is such that it dominates all surround- 
ing space, even to the iixed stars, so that a body at the 
distance of our nearest stellar neighbor, a Centauri, 
which is more than 200,000 times remoter than the sun, 
could free itself from the solar attraction only by dart- 
ing away with a velocity of more than 300 feet per sec- 
ond, or over 200 miles an hour; unless animated by a 
greater velocity than this, it would move around the 
sun in a closed orbit — an ellipse of some shape, or a 
circle — with a period of revolution which, in the smallest 
possible orbit, would be about 31,600,000 years, and if 
the orbit were circular, would be nearly 90,000,000. 
We say it would revolve thus — that is, of course, unless 

* The calculation of the sun's mass, from the data given, proceeds aa 
follows : Let M = the sun's mass, and m that of the earth ; R — the dis- 
tance from the earth to the sun, and r the mean radius of the earth ; T, 
the length of the sidereal year, reduced to seconds ; and \ g the distance 
a body falls in a second at the earth's surface. Now, the distance the 
earth falls toward the sun in a second, or the curvature of her orbit in a 

second, is equal to — ^^ (about 0-119 inch). Hence, by the law of gravita- 

27r^R m ]\r Att^rA 

tion, \ q '. ' = — : — whence, M = m I -„ |. 
■> ^J T2 r2 R2, ' \r'r^g] 

In this formula make 7r = 3*14159; R, 92,900,000 miles; T =31,- 
558,149-3 seconds ; r — 3,958*2 miles ; and \y - 0-0061035 mile (16*113 
feet), and we shall get the result given in the text, viz., M = 330,000 m 
(nearly). 



DISTANCE AND DIMENSIONS OF THE SUN. 41 

intercepted or diverted from its course by the influence of 
some other sun, as it probably would be. And we may 
notice here that in many cases certainly, and in most 
cases probably, the stars are flying throng] i space at a far 
swifter rate, wdth velocities of many miles per second. 

As for the attraction between the sun and earth, it 
amounts to thirty-six hundred quadrillions of tons : in 
figures, 36 followed by seventeen ciphers. On this 
point we borrow an impressive illustration from a care- 
ful calculation by Mr. C. B. Warring. We may imagine 
gravitation to cease, and to be replaced by a material 
bond of some sort, holding the earth to the sun and 
keeping her in her orbit. If now we suppose this con- 
nection to consist of a web of steel wires, each as large 
as the heaviest telegraph-wires used (No. 4), then to 
replace the sun's attraction these wires would have to 
cover the whole sunward hemisphere of our globe about 
as thickly as blades of grass upon a lawn. It would re- 
quire nine to each square inch. Putting it a little dif- 
ferently, the attraction between the sun and earth is 
equal to the breaking strain of a steel rod about 3,000 
miles in diameter. 

If we calculate the force of gravity at the sun's sur- 
face, which is easily done by dividing its mass, 330,000, 
by the square of 109^ (the number of times the sun's 
diameter exceeds the earth's), we find it to be 27i times 
as great as on the earth ; a man who on the earth would 
weigh 150 pounds, would there weigh nearly two tons ; 
and, even if the footing were good, would be unable to 
stir. A body which at the earth falls a little more than 
16 feet in a second would there fall 443. A pendulum 
which here swings once a second would there oscillate 
more than five times as rapidly, like the balance-wheel 
of a watch — quivering rather than swinging. 



42 THE SUN. 

Since the sun's volume is 1, 300,000 times that of the 
earth, while its mass is only 330,000 times as great, it 
follows at once that the sun's average density (found by 
dividing the mass by the volume) is only about one 
quarter that of the earth. This is a fact of the utmost 
importance in its bearing upon the constitution of this 
body. As we shall see hereafter, we know that certain 
heavy metals, with which we are familiar on the earth, 
enter largely into the composition of the sun, so that, 
if the principal portion of the solar mass were either 
solid or liquid, its mean density ought to be at least as 
great as the earth's ; especially since the enormous force 
of solar gravity would tend most powerfully to compress 
the materials. The low density can only be accounted 
for on the supposition, which seems fairly to accord 
also with all other facts, that the sun is mainly a ball of 
gas, or vapor, powerfully condensed, of course, in the 
central portion by the superincumbent weight, but pre- 
vented from liquefaction by an exceedingly high tem- 
perature. And, on the other hand, it could be safely 
predicted on physical principles that so huge a ball of 
fiery vapor, exposed to the cold of space, would present 
precisely such phenomena as we find by observation of 
the solar surface and surroundings. 



CHAPTEE 11. 

METHODS AND APPARATUS FOR STUDYING TEE SURFACE OF 

THE SUN. 

Projection of Solar Image upon a Screen. — Carrington^s Method of de- 
termining the Position of Objects on the Sun's Surface.—Solar Pho- 
tography.— Photoheliographs.—Janssen's Photographs. — Telescope 
with Silvered Object-Glass.— nerschePs Solar Eyepiece—The Polar- 
izing Eyepiece. 

The heat and light of the sun are so intense that 
peculiar instruments and methods are necessary for the 
observation of his surface. The appliances used in the 
study of the moon, planets, and stars will not answer 
at all for solar work. 

A very excellent method of proceeding where the 
object is to secure a general view of the sun, without 
regard to delicate detail, and to determine easily and 
rapidly the positions of spots and other objects on the 
sun's disk, is to project his image upon a sheet of card- 
board by means of a telescope. 

For this purpose things are arranged as indicated in 
the figure. The sheet of paper upon which the image 
is to be thrown is supported in front of the eyepiece by 
a light framework attached to the telescope. The dis- 
tance of the screen from the eyepiece depends upon the 
size of image desired and the power of the eyepiece ; a 
diameter of from six inches to a foot being generally 
most convenient. Another screen is usually fitted on 



44 



THE SUK 



the object-glass end of the telescope to balance the first, 
and shade it from all light except that which has passed 1 
through the instrument. If the apparatus is to be used j 
to determine the position of spots on the sun, the sur- 
face which receives the image must be carefully ad- 
justed so as to be perpendicular to the optical axis of ^ 
the telescope. 

Fig. T. 




To determine the position of objects on the sun's 
disk, Carrington used two lines, ruled at right angles to 
each other upon the screen, and set at an angle of about 
45"" with the north and south line or hour-circle. The 
observations needed to determine the place of a spot 
on the sun's disk then consist merely in noting with a 
watch as accurately as possible the four moments at 
which the edge of the sun's image crosses the two lines 
(the telescope being, of course, firmly fixed during the Ai 
whole time), and the two moments when tlie spot passes ■ 



METHODS FOR STUDYING THE SURFACE OF THE SUN. 45 

them. From these six observations, with the help of 
the data given in the ahnanac, the distance and direc- 
tion of the spot from the sun's center may readily be 
calculated by formulae which would hardly be suited to 
these pages, but which may be found in the monthly 
notices of the Royal Astronomical Society, vol. xiv, 
page 153. Fig. 8 illustrates this arrangement. 

Fio. 8. 



Another method, more convenient as involving no 
calculation, but less accurate, is to use Mr. A. Thom- 
son's " Charts for Sun-spot Observations," which are 
given in Sir Robert Ball's " Atlas of Astronomy " 
(Appletons, New York). Both these methods require, 
however, the use of a telescope equatorially mounted. 
With an instrument not so mounted, fairly good results 
may be obtained by drawing upon the screen a circle 
with a diameter about half tliat of the field of view, and 
noting the instants when the edge of the sun becomes 
tangent to the circle, and when the spot crosses it. 

With a small telescope thus fitted up, one is in a 
position to make observations of real value as to the 
number, position, and motions of the solar spots. Oc- 
casionally, also, when the air happens to be in good con- 
dition, a considerable amount of detail can be made out 
by this method in the spots and upon the solar surface 
generally. The darkening of the edge of the sun, 



46 THE SUN. 

caused by the absorption of the solar atmosphere, ' is 
very noticeable, and the faculse are conspicuous. One 
great advantage of the method is, of course, that several 
persons can thus observe together. A teacher, for in- 
stance, can in this way exhibit to a class of a dozen all 
the principal features of the sun's surface, and be sure 
that they all see the things he desires them to notice. 

Should any amateur happen to find upon the surfs 
disk a small, round spot, which he has reason to think 
is an intra-Mercurial planet, a few observations of the 
sort indicated above, repeated at intervals of some min- 
utes, would settle the question immediately, and give a 
reasonably accurate determination of the rate and direc- 
tion of movement. 

If the instrument has an equatorial mounting and 
clockwork, so that the image remains apparently sta- 
tionary upon the screen, a very satisfactory tracing can 
be made upon paper ruled in squares, showing pretty 
accurately the position and magnitude of all visible 
spots, in a form suitable to file away for reference. The 
observations of Carrington's great work upon the solar 
spots were for the most part made in this manner. 

Of late years photography has been extensively util- 
ized for observations of this sort. The apparatus con- 
sists of a telescope fitted with a camera-box in place of 
an eyepiece, and with an arrangement for producing an 
instantaneous exposure of the sensitive plate to the 
solar rays. 

Since, in the ordinary achromatic telescope, the rays 
which are most effective in photographic action do not 
come to a focus at the same point as those which most 
strongly affect the eye, such an instrument, however 
perfect visually, will not give sharp photographic im- 
pressions. It is necessary, for the best photographic 



I 



METHODS FOR STUDYING THE SURFACE OF THE SUN. 47 

results, to use object-glasses whose corrections are cal- 
culated expressly for the purpose. Mr. Rutherfurd, of 
New York, seems to have been the first to appreciate 
this, and to construct an instrument specially designed 
for astronomical photography. To this end, disdaining 
all compromise, he did not hesitate to sacrifice delib- 
erately the visual excellence of an exquisite object-glass 
of thirteen inches diameter, by altering its curves so as 
to produce the most perfect actinic correction ; and he 
was rewarded by a success until recently unequaled as 
regards the perfection of the pictures obtained. Some 
of his photographs of the sun and moon, obtained about 
1866, rival in sharpness and detail the drawings of ac- 
complished observers. 

Another and simpler method of obtaining the de- 
sired corrections, originally tried by Mr. Rutherfurd 
and rejected as not absolutely the best possible, has 
been revived and used by Cornu, of Paris. It consists, 
not in regrinding the two lenses which compose the 
object-glass, but merely in separating them slightly — 
haK an inch or so for an instrument of ten-feet focus. 
The approximate correction, thus produced, gives excel- 
lent results, and the instrument is not spoiled for other 
work, since it requires only a few minutes to restore 
the glasses to their visual adjustment. 

In a reflecting telescope there is, of course, no difii- 
culty of this sort, since rays of different wave-length and 
color are not dispersed by reflection as by refraction. 
Other and still more serious diflSculties, however, exist, 
depending upon the extreme sensitiveness of the reflec- 
tor to the distorting influence of variations of tempera- 
ture ; so that, hitherto, reflectors have not equaled re- 
fractors in the excellence of their photographic work. 
They have been employed with very good success, how- 



48 



THE 8UN. 



ever, on several oecasions for the photography of solar, 
eclipses. 

With telescopes of considerable size the picture is 
generally formed directly at the focus of the object- 
glass without further enlargement. This is the case 
with the pictures made by Mr. Rutherfurd, in which 
the diameter of the sun's image is about If inch. 



Fio. 9. 




K F,W PlIOTOIIELIOGRAPn. 



Copies of the negatives are afterward made if desired 
on a larger scale. In smaller instruments, such as the 
well-known photoheliograph of the Kew Observatory, 
an enlarging eyepiece is used, so constructed as to dis- 
tort as little as possible the image formed by the object- 
glass while magnifying it to a diameter of three or four 



METHODS FOR STUDYING THE SURFACE OF THE SUN. 49 

inches. In this instrument, of which we give a figure, 
the diameter of the object-glass is only 3|- inches, and 
its focal length 50 inches ; the tube, instead of being 
conical as usual and larger at the object-end, is made 
pyramidal and larger at the bottom, in order to ac- 
commodate the plate-holder more conveniently. The 
w^hole is mounted equatorially, and driven by clockwork. 
It was constructed in 1857, under the directions and 
after the designs of Mr. De La Rue, and proved itself a 
most efiicient and excellent instrument. A number of 
other very similar instruments have since been made 
with slight improvements. Those employed by the 
English and Russian parties in their photographic opera- 
tions at the transit of Venus in 1874 were of this type. 
So also w^ere those of the German parties, except that 
they had considerably larger telescopes, with apertures 
of from six to eight inches. The photoheliograph now 
used at Greenwich in maintaining the daily record of 
the sun's surface is one of the Transit of Venus instru- 
ments, having a four-inch object-glass, and giving a solar 
image four inches in diameter. It is mounted equato- 
rially with clockwork. The instruments at Mauritius 
and Dehra Dun are similar. More recently Greenwich 
has come into possession of a nine-inch photoheliograph, 
which is used in connection with the other to obtain 
pictures on a larger scale. 

The sunlight is so powerful that the exposure of 
the plate has to be made practically instantaneous. The 
apparatus by which this is effected varies greatly in de- 
tail in instruments of different types, but in all cases 
consists essentially of a slide, carrying in it a slit of 
adjustable width and capable of being shot across in 
front of the sensitive plate by a strong spring. At the 
moment of exposure a trigger or telegraphic key is 



50 



THE SUN. 



touched by the operator, and the slide, previously drawn 
back and locked by suitable mechanism, is released, and 
in its flight allows the rays to gleam through the aper- 
ture for a time, which in different instruments varies 
from yi^ to -g^oVo" ^^ ^ second, according to the size of 
the instrument, the sensitiveness of the collodion, and 
the clearness of the atmosphere. 

We give a figure of Yogel's exposure-slide, which is 
perhaps as good as any. M is an electro-magnet, which, ] 



Fig. 10. 




Vogel's Exposure-Slide. 

on the touch of a telegraph-key in the observer's hand, 
attracts the armature B, thus releasing the catch 0, and 
allowing the spring S, by the intervention of the cord 
and pulley, to draw the slide containing the slit A swift- 
ly across the orifice through which the rays enter the 
camera. 

The character of the picture produced depends very 
greatly upon the proper timing of the exposure. If 
the intention be to secure an image of the sun with 
hard, firm edges from which measurements can be made 
to determine the position of objects on the solar disk — 



METHODS FOR STUDYING THE SURFACE OF THE SUN. 51 

as was the case at the transit of Venus — then a relatively 
long exposure is needed ; but it is to be remembered 
that the diameter of the sun's image increases very per- 
ceptibly with lengthening exposure, so that this diam- 
eter can never be safely used to furnish the scale of 
measurement. If, on the other hand, what is desired 
is a picture full of detail, showing the faculse and the 
structure of the spots, the exposure must be greatly 
shortened by narrowing the slit or giving the slide a 
greater velocity ; and it must be added, unfortunately, 
that the exposure which brings out perfectly the cen- 
tral portions of the disk is altogether too short for the 
portions near the limb, where the actinic power is very 
greatly diminished. 

This circumstance detracts considerably from the 
value of the photographic method. The skilKul 
draughtsman can show in the same picture details dif- 
fering to any extent in intensity, while the photograph 
is, so to speak, limited to the reproduction of only one 
certain class of details at a time. Still we can always 
be sure that, whatever a photograph does show, is an 
autographic representation of fact, and not a figment of 
the imagination. This is not the case with drawings ; for 
it is remarkable how widely two conscientious artists 
will differ in their representations of the same object, 
seen by both with the same telescope, and under the 
same circumstances. As an accurate record of the num- 
ber, position, and magnitude of the solar spots at any 
given time, the photograph is, of course, unexception- 
able. 

Such a record was maintained by the Kew photo- 
heliograph for fourteen years — from 1858 to 1872 — 
when the work was discontinued. An almost equally 
important series of photographs was kept up for many 



52 THE SUN. 






years at Wilna, in Russia, until the burning of the ob- 
servatory in 18Y7. Since 1873 the Kew series has been 
continued at Greenwich, at least two pictures being 
taken every day when the wxather will permit, and 
more than two if anything of special interest demands 
it. This Greenwich record is supplemented by the neg- 
atives taken at Dehra Dun, in India, and at Mauritius. 
Taken together with the Green w^ich plates, they furnish 
a practically continuous record of the condition of the 
solar surface. At the same time there are occasional 
breaks, which might be remedied if we had one or two 
photoheliographs on the American side of the Atlantic. 

Of late, Janssen, at tlie new French physical ob- 
servatory at Meudon, has carried solar photography to 
a point far beyond any previous attainment. He has 
accomplished it mainly by utilizing the fact that there 
exists in the spectrum, near the Fraunhofer line G, a 
narrow band of rays which possess a photographic ac- 
tivity upon the salts of silver much more intense than 
that of any other portion of the spectrum. It is so 
intense, indeed, that if the exposure be very short and 
properly regulated, the effect is practically the same as 
if the sunlight w^ere monochromatic, consisting of these 
rays alone: any defect in the color-correction of the 
object-glass is rendered almost harmless. This makes 
it possible to use an ordinary achromatic object-glass, 
roughly corrected for photographic work by merely 
separating the lenses a trifle, according to Cornu's plan. 

With a five-inch telescope and a suitable enlarging 
lens, Janssen produces pictures even half a metre in 
diameter, and of extreme perfection in their delinea- 
tion of the details of the solar surface. The exposure, 
ranging from ^^-g- to ytoit ^^ ^ second, according to the 
clearness of the air and the altitude of the sun, is effected 



METnODS FOR STUDYING THE SURFACE OF THE SUN. 53 

by a slide closely resembling Vogel's. The impression 
obtained is very feeble, and requires prolonged and 
careful development ; but, when at last fairly brought 
out, is every way admirable. Some very interesting 
results, which we shall deal with later, have already 
been deduced from his plates. 

Photography, however, is not adequate as yet to the 
study of the most delicate details of the solar surface. 
For this purpose nothing can take the place of ocular 
observation by experienced and skillful observers, armed 
with powerful telescopes and suitable appliances, and 
on the watch for the few favorable moments when the 
atmospheric conditions will permit successful work. 

The instrument must be provided with some form 
of solar eyepiece expressly adapted to the purpose. 
The old-fashioned way was to use an ordinary eyepiece, 
fitted with a dark glass next the eye. If the whole 
aperture of a telescope of any size is used, the heat at 
the focus is so great as to endanger the lenses, and ac- 
cordingly it was customary to " cap down " the object- 
glass — i. e., to put on a cover with a small hole in the 
center, so as to reduce the aperture to two or three 
inches. In this way, of course, the heat and light are 
easily diminished to almost any extent, but the defini- 
tion is greatly injured. According to well-known opti- 
cal principles, the image of a luminous point is not a 
point, even in an absolutely perfect telescope, but, in 
consequence of the so-called " diflfraction " due to the 
interference of light, becomes a small disk, surrounded 
by a series of concentric luminous rings ; the smaller 
the aperture of the telescope, the larger the disk with a 
given magnifying power. Similarly, the image of a 
luminous line is not a line, but a stripe of determinate 
width with fringes on each side. It is easy to see. 



54 THE SUN. 

therefore, that a telescope of small aperture can not 
possibly be made to show as delicate details as one of 
larger diameter, and, to get the best results in examin- 
ing the surface of the sun, we must find some way of 
diminishing the light and heat without cutting down 
the diameter of the object-glass (or mirror, if we are 
using a reflecting telescope). 

A reflecting telescope whose mirror is of unsilvered 
glass effects this very beautifully. The unsilvered sur- 
face reflects only about -^^ of the incident light and 
heat, and although the resulting image is still too bright 
for the unprotected eye, the heat is not troublesome, 
and only a very thin shade-glass is needed. Another 
excellent method is to silver by Liebig's or some analo- 
gous process the front surface of the object-glass of a 
refractor. The silver film can be deposited of such a 
thickness as to allow any desired percentage of the 
light to pass, while the rest is reflected and not allowed 
to enter the instrument at all. The image formed in 
this way is slightly tinged wdth blue, but is "beautifully 
sharp and steady, there being a great, advantage in pre- 
venting the heating of the air in the telescope-tube, 
which occurs with every other form of instrument. 
The telescopes employed by the French parties in the 
observation of the transit of Venus in 1874 were pre- 
pared in this way. With its great advantages, however, 
the method has on the w^hole quite as great disadvan- 
tages, as was evident at Saigon, where clouds were so 
thick that nothing could be seen through the silver film, 
and the observer had to rub it off with a cloth before 
he could do anything. Then, of course, a telescope pre- 
pared in this way can not be used for any other pur- 
pose. The common practice, therefore, is, not to adapt 
the instrument for solar observation by doing anything 



METHODS FOR STUDYING THE SURFACE OF THE SUN. 55 

to its object-glass or mirror, but to accomplish the 
desired result by some moditication or accessory of the 
eyepiece. 

One of the best known and most generally useful 
eyepieces is that devised by Sir John Herschel, and 
bearing his name. It is represented in Fig. 11, which 
gives a section of it. The light entering at O encoun- 




1 -. 



Solar Eyepiece. 



ters a prism of glass, whose first surface is placed at an 
angle of 45°. The greater part of the light, something 
over 1^1, passes through the prism, emerging perpen- 
dicular to its second surface, and goes out through the 
open end of the tube ; the reflected light, about -^-^ of 
the whole, is thrown upward through the eyepiece 
proper, A B, which is precisely the same as ordinarily 
used. In this v/ay most of the light and heat are got 
rid of; too much, however, still passes the lenses for 
the eye to bear, and it is necessary to use a shade- 
glass ; but this may be very light. The brightness of 
the sun varies so much at different altitudes and under 
different conditions of the atmosphere, that it is de- 



56 



THE SUN. 



sirable to have the thickness of the shade-glass adjust 
able. This is easily managed by using a long, thini 
wedge of dark glass, compensated by a corresponding 
wedge of ordinary glass, and set in a proper frame, as 
represented in Fig. 12. The shade-glass should not be 

Fig. 12. 




colored, but of neutral tint, so that objects on the sun's 
surface may be seen of their proper hue. The glass 
known as " London smoke " very nearly fulfills this 
condition, and with a shade of this material the appa- 
ratus is exceedingly satisfactory, and quite sufficient 
for all ordinary work. 

Still finer results, however, may be obtained with 
more complicated and expensive " helioscopes," as they 
are called, which by means of polarization reduce the 
light to such a degree that no shade is needed, and, 
moreover, enable us to graduate the light as we please by 
merely turning a milled head. There are several forms 
of the apparatus : we give a figure of one constructed by 
Merz, slightly modified,* which is perhaps as convenient 
and effective as any. The light entering at A first en- 

* The modification consists in substituting the prisms P ^ and P '^ for 
simple reflectors of black glass, which are very api to be broken by the 
heat of the sun's image. 



METHODS FOR STUDYING THE SURFACE OF THE SUN. 57 

counters the surface of a prism, P*, set at the polar- 
izing angle ; about i|- of the light passes through the 
prism, emerging perpendicular to its rear surface, and, 
being rejected : about -^ is reflected and polarized by 
the reflection. The reflected ray next strikes the sur- 
face of a second prism, P ^, and here a considerable por- 
tion of the remaining light is thrown away. That which 



Fig. 13. 




Meez's Helioscope. 



is left is reflected into the upper portion of the eyepiece 
parallel to its original direction, through an opening in 
the top of the circular case in which the two prisms 
are mounted. The upper case is attached to the lower 
in such a manner that it can be turned around the line 
C D as an axis. It contains two plane mirrors of black 
glass, placed as shown in the figure. With things in 



58 THE SUN. 

the position indicated, a beam of considerable strength 
would reach the eye at B — so strong, in fact, as to be 
painful ; and the same would be the case if the upper 
piece were turned 180°, bringing the mirrors into the 
position shown by the dotted lines, with the issuing ray 
in the prolongation of the incident. But, by turning 
the upper piece one quarter of a revolution, the issuing 
ray can be entirely extinguished, and, by turning it less 
or more than 90°, the intensity of the light can be con- 
trolled at pleasure. As no shade-glass is used, every- 
thing is seen of its proper tint. Another advantage is, 
that there is no such disturbance of the orientation of 
the solar image as happens with every form of diagonal 
eyepiece. North, south, east, and west fall in their 
usual and natural places — a matter of some importance 
as regards the convenience of observation. 

Still other forms of helioscopic eyepiece depending 
upon polarization have been devised by Secchi, Lang- 
ley, Christie, Pickering, and others, each with its own 
peculiar advantages ; our limits, however, forbid more 
extended treatment of the subject. We add merely 
that in some cases, as in the study of the internal struc- 
ture of sun-spots, it is found very advantageous to 
adopt the device of Dawes, and limit the field of view 
by a minute diaphragm made by piercing a card or 
plate of ivory with a hot needle ; thus excluding the 
light from any portion of the sun's surface except that 
under immediate observation. 



II 



■ 



CHAPTER III. 

TEE SPECTROSCOPE AND THE SOLAR SPECTRUM, 

The Spectrum and Fraunhofer's Lines. — The Prismatic Spectroscope ; 
Description of Various Forms and Explanation of its Operation. — 
The Dififraction Spectroscope. — The Concave-Grating. — Analyzing 
and Integrating Spectroscopes. — The Telespectroscope and its Ad- 
justment. — The Spectrograph. — Explanation of Lines in the Spec- 
trum. — Kirchhoff's Researches and Laws. — The Sun's Absorbing 
Atmosphere and Reversing Layer. — Elements present in the Sun. — 
Lockyer's Researches and Hypothesis. — Basic Lines. — Dr. H. Dra- 
per's Investigations as to the Presence of Oxygen in the Sun. — 
Schuster's Observations. — Eifect of Motion upon Wave-Length of 
Rays and Spectroscopic Determinations of Motion in Line of Sight. 

Ever since the time of Newton it has been known 
that a beam of white light is decomposable into its con- 
stituent colors by passing it through a prism, and, under 
certain circumstances, the result is a rainbow-tinted band 
or ribbon, which has been called the solar spectrum. 
In this spectrum Wollaston, in 1802, discovered certain 
dark shadings, and in 1814 Fraunhofer again and inde- 
pendently discovered the same thing; and he so im- 
proved his apparatus and method of observation as to 
get not merely indefinite shadings, but clear, sharp 
lines, of which he made a map, assigning designations 
to many of the principal ones. Indeed, these markings 
of the solar spectrum bear his name to this day. 

He, however, could not account for them, further 
than to show that they did not originate in his instru- 
ment nor in the earth's atmosphere; and it was not 



60 THE SUN. 

until the publication of the researches of Kirchhoff and 
Bunsen, in 1859 and 1860, that the scientific world 
came to appreciate their meaning and importance. 

We speak of the work of Kirchhoff and Bunsen as 
epoch-making, and such was certainly the case. At 
the same time tlie secret of the solar spectrum had been, 
in part at least, divined before by Stokes, Thomson, 
and Angstrom ; the latter especially, whose memoir, 
published in 1853, would certainly have obtained a 
high celebrity if it had appeared in French, Englisii, 
or German, instead of Swedish. Swan and Zantedeschi 
had also given to the spectroscope nearly its present 
form ; and a number of other investigators, among 
w^hom Sir John Herschel, Wheatstone, Foucault, and 
J. W. Draper deserve special mention, had each con- 
tributed something important to the foundations of the 
new science, for such it has proved to be. The study 
of spectra has opened a new world of research, and 
added some such reach to our pliysics and chemistry as 
the telescope brought to vision. 

Of course, any extended discussion of the instru- 
ments, principles, and methods of spectroscopy would 
be inconsistent with our limits : we can only treat the 
subject very briefly. 

First, then, as to the instrument. It consists usually 
of three parts : the collimator so called ; the light-ana- 
lyzinoj apparatus, which is sometimes a prism or train 
of prisms, and sometimes a diffraction grating ; and the 
view-telescope. Figure 14 shows the construction of a 
single-prism spectroscope, and the course of the rays of 
light through it. The collimator is simply a telescope 
without an eyepiece, and having in the place of the eye- 
piece a narrow slit. This slit is placed exactly at the 
focus of the object-glass of the collimator, so that rays 



THE SrECTROSCOPE. 



61 



from each point of the slit become parallel beams after 
passing the lens, and a person looking through the ob- 
ject-glass, at the slit, sees it precisely as if it were an 
object in the sky. Optically, the slit of the collimator 



Fig. 14. 




Abrangement of Prismatic Spectroscope. 

is thus removed to an infinite distance ; while, mechani- 
cally, it is still at the fingers' ends, within reach of ma- 
nipulation and adjustment. The collimator, however, is 
not essential. Fraunhofer's work was all done with 
light admitted through a slit in the window-blind at a 
distance of twenty or thirty feet — a much less con- 
venient arrangement, as is at once evident. 

The view-telescope, which, however, is no more essen- 
tial than the collimator, is usually a small telescope with 
an object-glass of the same size as that of the collimator, 
and magnifying from five to twenty times. Generally, 
the collimator and telescope of astronomical spectro- 
scopes are from three quarters of an inch to two inches 
and a half in diameter, and from six to forty inches long. 

The light, after passing the slit and object-glass of 
the collimator, next strikes the prism or grating, and 
these two things — the slit and the prism or grating — 



G2 THE SUN. 

are really all that is essential. In the case of a prism 
(which must be set with its refracting edge parallel to 
the slit) the rays are bent out of their course, as shown 
in the figure, and enter the view-telescope, placed at 
tlie proper angle to receive them. Suppose, now, for 
a moment, that the light admitted at the slit is strict- 
ly homogeneous — say red. The eye at the view-tele- 
scope would then see a red image of the slit, corre- 
sponding precisely in form and proportions to the slit 
itself, widening or narrowing as the slit is acted on 
by its adjusting screw. If instead of a slit the open- 
ing had some other form, as an arc of a circle, a tri- 
angle, or a square, the image seen would imitate it, 
always having the same color as the light admitted. 
Suppose, again, that the light is not homogeneous, but 
consists of two kinds mixed together — say red and yel- 
low. Yiew^ing the slit directly, without the spectro- 
scope, one would only see a single orange-colored im- 
age ; but with the spectroscope one w^ould see two 
w^idely separated images, one of them red, the other 
yellow. This is because the prism refracts the two 
kinds of light differently, so that after the rays have 
passed the prism they strike the object-glass of the view- 
telescope in different directions, and then make images 
in different places. If the light is composed not of two 
kinds only, but many, the images will be numerous, 
ranged side by side like the pickets of a fence ; and if, 
as in the case of a candle-flame, the light emitted con- 
tains an indefinite number of tints, then the slit-images, 
placed side by side, will coalesce into a continuous 
band of color. If, in the candle-light, certain kinds of 
light are specially abundant, then the corresponding 
slit-images will be more brilliant than their neighbors ; 
and if, as is usually the case, the slit be narrowed to a 



* 



THE SPECTROSCOPE. 



63 



line, these slit-images will become hright lines in the 
spectrum — lines only because the slit is itself a line, 
which, of course, is the best form to give the light-ad- 
mitting aperture, in order that the different images may 
overlap and interfere as little as possible. 

If any kinds of light be wanting, then the corre- 
sponding images of the slit will be missing, and the 
spectrum will be marked by dark bands or lines. 

Fig. 15. 




Eunsen's Spectkoccopi: 



The cut (Fig. 15) shows the actual appearance of 
what is known as the chemical spectroscope, ordinarily 
used in laboratories. Besides the collimator A, and the 
telescope B, it has a third tube C, which carries a fine 
scale photographed on glass at the end farthest from 
the prism. There is a lens in the tube at the end next 
the prism, so that the observer at the telescope sees this 
scale running across the field of view at the edge of the 
spectrum, and thus has the means of noting accurately 



64 



THE SUN. 



the position of any lines he may find. This arrange- 
ment is due to Bunsen. 

It is often desirable to obtain a greater separation of 
the different colors — '* dispersion," to use the technical 
term — than a single prism would produce. In this case, 
the rays after passing through the first prism may be 
transmitted through a second and a third, and so on, 
until they reach the view-telescope. With prisms as 
commonly made, it is difficult to use more than six in 
this manner, but it is possible by reflection properly 
managed to return the rays through a second prism- 
train connected with the first, so as to get the virtual 
effect of from ten to twelve prisms. The instrument 
figured on pages 78 and 190, and used for observation 
of the solar prominences, is of this kind. 




^ COMPOUND PRISM. ^ 




I 



DIRECT-VISION PRISM 



Another way is to use a compound prism, so called, 
composed of a very obtuse-angled prism, A B E, of 
some highly dispersive material, usually heavy flint 
glass, flanked by two prisms of lighter glass with 
their refracting angles reversed. Prisms of this kind 



THE SPECTROSCOPE. 65 

can be made of much higher dispersive power than 
simple prisms, and of course a smaller number will 
answer the same purpose. By properly proportion- 
ing the angles C A E and E B D, it is possible to make 
the yellow rays of the spectrum pass through without 
change of direction, while still retaining a considerable 
dispersion. An instrument with prisms of this kind is 
called a " direct-vision " spectroscope, and in some cases 
is much more convenient than the other forms. 

Thollon has recently constructed compound prisms 
having the dense glass prism replaced by a chamber 
filled with carbon disulphide, v/hich possesses an enor- 
mous dispersive power ; with a train of these prisms he 
has obtained views of the spectrum only equaled by the 
performance of the best diffraction gratings. A dis- 
persion equal to that of thirty or forty prisms of an 
ordinary spectroscope is easily reached. The behavior 
of these disulphide prisms is, however, far from satis- 
factory for general work, since they are extremely sen- 
sitive to small changes of temperature, which cause 
irregular refractions in the liquid, and destroy the defi- 
nition. 

We have used the expression, the dispersive power 
of thirty or forty prisms ; but that is very indefinite, 
because the dispersive powder of a spectroscope depends 
upon its linear dimensions as well as the kind and num- 
ber of prisms and is proportional to the dimensions. 
That is to say, if a given spectroscope has the size of its 
prisms, and the diameter and focal lengths of its col- 
limator and telescope doubled, retaining, however, the 
former slit and eyepiece, its dispersive power will be 
doubled by the change. Thus a large single-prism in- 
strument may equal in working power a small one of 

many prisms. 
6 



I 



66 THE SUN. 

Lord Rayleigh lias shown that the resolving power 
of a spectroscope, constructed with prisms of. a given 
substance, depends upon the length of the route pur- 
sued by the rays of light in traversing them. 

As has been said, a diffraction grating may replace 
the prism in a spectroscope. This diffraction grating 
is merely a system of close, equidistant, parallel lines 
ruled upon a plate of glass or polished speculum-metal. 
The closer the lines, the greater the dispersion pro- 
duced ; the larger the ruled surface, the more light is at 
the observer's disposal, provided the collimator and 
view-telescope are large enough to utilize the whole 
ruling. The greater the total number of lines, the 
higher the resolving power of the grating, or power of 
separating close lines in the spectrum. 

It hardly needs to be said that the making of a sat- 
isfactory grating is by no means an easy matter. To 
w^ork a surface optically accurate, and to rule it with 
perfectly straight equidistant paralled lines, 20,000 to the 
inch or so, and all of the same w4dth and depth, is one 
of the most delicate and difficult of mechanical opera- 
tions. The first that were fit for spectroscopic use were 
made in this country, about 1S71, by Mr. Rutherfurd, 
of New York, upon a ruling machine devised and con- 
structed by him for the purpose : they were first act- 
ually applied in solar spectroscopy by the writer, in 
1873. In 1881, when the first edition of this book ap- 
peared, some very fine ones had already been produced, ■ 
with ruled surfaces nearly two inches square, carrying 
17,280 lines to the inch. One of these, still in constant 
use in the Princeton Observatory, is especially excellent, 
and except in size is hardly inferior to the magnificent 
specimens now produced at Baltimore by Professor 
Rowland's wonderful machine, which at present, with 



THE SPECTROSCOPE AXD THE SOLAR SPECTRUM. 67 

its newest improvements and refinements, is quite with- 
out a rival, and almost ideally perfect. Ever since 1882 
it has been turning oflf gratings of admirable excel- 
lence : the largest have a ruled surface of about o^ by 4 
inches, with more than 100,000 lines (20,000 to the 
inch). These have been widely distributed among sci- 
entific workers, and it is not too much to say that all 
the recent important researches upon the solar spec- 
trum (Thollon's alone excepted) owe their success to 
Rowland's gratings. 

An explanation of the way in which diffraction 
spectra are produced by a grating lies beyond our 
scope ; for this the reader is referred to any good trea- 
tise on optics. We merely state, in passing, that diffrac- 
tion has nothing to do with refraction^ but depends 
upon the fact that the ether waves, of which light con- 
sists, under certain circumstances " interfere " with each 
other and produce brilliant effects of color. We say 
spectra^ because, while a prism gives but one spectrum, 
a grating gives many, and of different degrees of dis- 
persion, which is often a matter of much convenience. 
Of course, no one of the spectra is as brilliant as if it 
were the only one, but with sunlight this is a matter of 
small consequence. Besides, it is possible, by giving the 
proper shape to the diamond point which rules the lines 
and properly regulating the depth of cut, to produce 
gratings in which most of the light shall be concen- 
trated in a single one of the spectra at the expense of 
the rest. 

A good grating combined with a suitable collimator 
and telescope constitutes a spectroscope which for most 
solar work is incomparably superior in power and con- 
venience to any prismatic instrument of similar dimen- 
sions, so that as a matter of fact the grating-spectro- 



68 THE SUN. 

scope has almost superseded the other in this sort of 
work. 

Fig. 17 shows the arrangement of the different parts 
of such an instrument. The collimator and view-tele- 

FiG. 17. 



sL/r 





Diffraction Spectroscopb, 



scope are placed with their object-glasses close together, 
and their tubes making as small an angle as possible, 
consistently with keeping the grating at a manageable 
distance, since both collimator and telescope must be 
pointed at the center of the grating. The grating is 
mounted on a frame with an axis at A, so that it can] 
rotate in the plane of the dispersion, the ruled linei 
being parallel to this axis. The frame which carries 
the grating must be so constructed as to support it 
steadily and firmly, without the slightest strain, for it 
is essential to its good performance that the surface be 
strictly plane : an abnormal pressure of a single ounce 
at one of the corners will sensibly affect its perform- 
ance, and four ounces would bend the plate sufficiently 
to ruin the definition. 

As the different orders of spectra overlap each 
other (the red end of the second order spectrum over- 
lapping the blue of the third, etc.), it is sometimes neces- 
sary to separate them, and this can be done in a man- 
ner first suggested by Fraunhof er, by interposing between 



s 

i 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. G9 

the grating and view-telescope a single prism with its 
plane of dispersion perpendicular to that of the grating, 
the telescope being then inclined at the proper angle to 
receive the rays. A direct- vision prism in the eyepiece 
answers the same purpose, though less satisfactorily. In 
many cases a suitably colored shade-glass is sufficient. 

Fig. 18 is from a photograph of an instrument act- 
ually in use at Princeton for observations upon solar 
prominences. It is designed to be attached to a nine- 
inch equatorial, its collimator and view-telescope being 
each only about thirteen inches long, with a diameter 
of an inch and a quarter. The prism, P, is used only 

Fig. 18. 




Princeton Spectroscope. 



occasionally and can easily be removed. The telescope, 
T, is then lowered to the same level as the collimator, so 
as to be perpendicular to the lines of the grating. 

There is, however, a form of grating-spectroscope 



YO 



THE SUX. 



invented by Professor Rowland which uses not a flat 
but a concave grating, and dispenses with both the col- 
limator and the telescope. For certain researches, such 
as the mapping of the spectrum of the sun or of metal 
lie vapors, or for comparing together different spectra, 



Fig. 19. 




Concave Grating-Spectroscope. 



it is the most powerful and effective of all spectroscopic 
apparatus. The arrangement is that indicated in Fig. 
19. The grating, G, is mounted at one end of a stiff 
bar, C, at the other end of which is placed an eyepiece. 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 71 

Two pivots, one with its center directly under the cen- 
ter of tlie ruled surface of the grating, and the other 
at I, distant from the first by the radius of the 
spherical surface of the grating, connect the rod wdth 
two carriages w^hich ride upon the rails R and E^ 
These are firmly secured exactly at right angles to each 
other upon the two strong beams A and B, and at the 
point where the rails would meet is placed the slit S. 
With this arrangement, if I is made to slide along B 
either toward the right or left, G will slide along A ; and 
since the angle at S is a right angle, the three points, G, 
S, and I, will always be on the circumference of a circle 
whose diameter is G I. Under these conditions light 
passing through the slit and striking the grating will 
form a perfectly focused spectrum at I, which can be 
viewed with the eyepiece. 

If desired, a photographic plate can be placed at I 
and the spectrum photographed. If I is moved tow^ard 
the right it will run toward the red end of the spec- 
trum, and toward the violet if moved toward the left. 
With a six-inch grating the rod G I is usually from 
fifteen to twenty-five feet long, and the dispersion is 
tremendous. The apparatus is mounted in a large room 
perfectly darkened, and the sunlight is brought in by a 
heliostat mirror, through a suitably protected orifice. 
It ^vas with an apparatus of this kind that Professor 
Rowland made his great photographic map of the solar 
spectrum (page 78), and has studied the spectra of 
nearly all the chemical elements. The theory of the 
instrument is quite beyond our range : those w^ho are 
sufficiently versed in mathematics will find it given in 
the " Encyclopaedia Britannica," article " Wave-theory 
of Light," § 14. 

The prismatic and diffraction (or interference) spec- 



72 THE SUN. 

tra differ from each other to a certain extent, not, of 
course, in the order of colors or of lines, but in their 
relative distances. In the prismatic spectrum the red 
and yellow portion is much compressed, while the violet 
is greatly extended ; with the diffraction spectrum the 
reverse is the case ; the lines in the violet are crowded 
together, and those in the red are widely separated. 

In the diffraction spectrum the lines are almost per- 
fectly straight ; in the prismatic, generally more or less 
curved ; we say generally, because there are forms of 
high-dispersion spectroscope in which this curvature is 
corrected. This curvature is caused by the fact that 
the rays from the top and bottom of the slit do not 
meet the refracting surface at the same angle as those 
from the middle of the slit ; they are, therefore, differ- 
ently refracted ; in consequence, the slit-images of which 
the spectrum is built up are not straight, but distorted. 

We may add that the dark lines which often run 
lengthwise through the spectrum are merely due to 
roughnesses or particles of dust on the jaws of the slit. 
It is almost impossible to make and keep the edges of 
the slit so clean and smooth that lines of this sort will 
not appear when the opening is very narrow. 

The spectroscope may be used in two entirely dif- 
ferent ways : it may simply have its collimator pointed 
toward the source of light ; or a lens may be interposed 
between the slit and the luminous object, so as to form 
an image of the latter on the slit. 

In the first case, the instrument is said to be an in- 
tegrating spectroscope, because each point in the slit 
receives light from the whole of the luminous object, so 
that the spectrum is alike through its whole width, and 
represents the average light of the object — it hemps the 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 73 

whole, so to speak. In the second case, different parts 
of the slit are illuminated by light from different parts 
of the object ; the top of the slit gets the light from 
one point, the middle of the slit from another, and the 
bottom from a third. If, then, the lights emitted by 
the three points differ, their spectra will differ also, and 
the observer will find that different portions of the 
width of his spectrum will differ correspondingly — the 
upper portion will be unlike the middle, and the mid- 
dle will differ from the bottom. An instrument ar- 
ranged thus is called an analyzing spectroscope, because 
it enables us to determine separately the spectra of vari- 
ous portions of an object, and thus to analyze its consti- 
tution ; as, for instance, a sun-spot and its surroundings. 
For most purposes, especially astronomical, it is much 
the most satisfactory. Approximately the same end 
may be reached, in some cases, by placing the slit very 
near the luminous object, as in flame analysis, but it is 
usually much more convenient and better to use the 
lens. In astronomical work the object-glass of a large 
equatorial telescope is generally employed to form the 
image of the celestial object, and the spectroscope is 
attached at the eye-end of the telescope, the eyepiece 
being removed. The combined instrument is then often 
called a tele-spectroscope. Fig. 20, on the next page, 
represents the apparatus long used at the Dartmouth 
College Observatory. 

It is usually very important that the slit of the in- 
strument be precisely in the focal plane of the object- 
glass of the telescope for the rays especially under ex- 
amination. On account of the so-called " secondary 
spectrum" of the achromatic lens, this focal plane is 
quite different for the different colors, and the spectro- 
scope requires to be slid in or out, so as to vary the 



74 



THE SUN. 



distance of its slit from the great object-glass of the 
telescope according to circumstances. The same end 
may be obtained (less satisfactorily, however) by a sec- 
ond lens between the object-glass and the slit, and 
pretty near the latter. By moving this lens, the focus 



Fig. 20. 




Tele-Speotroscope. 



can be made to fall exactly on the slit. Neglect of this 
adjustment will make many of the most interesting and 
important spectroscopic observations quite impossible. 

As has been already mentioned, photography is often 
used in connection w^ith the spectroscope, and the name 






II 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. Y5 

spectrograph has been given to tlie combination instru- 
ment. Fig. 21 represents the spectrograph which is 
attached to the 23-inch equatorial of the Halsted Ob- 
servatory at Princeton. As shown in the figure, it uses 
a grating, but this can, at pleasure, be exchanged for a 
train of prisms. 

Some of the advantages of photography are self- 
evident — as, for instance, the quickness and accuracy 
with which a map of any portion of the spectrum can 
be produced, as contrasted with the tediousness of any 
drafting process. Then, too, since our modern plates 
admit of any desired length of exposure (instead of dry- 
ing up in a few minutes, as our old " wet plates " used 
to), they enable us to get satisfactory negatives of spec- 
tra far too faint to be visually observed. Finally, there 
is a long range of ultra-violet spectra composed of rays 
of too short wave-length and too high a " pitch " to be 
seen by the human eye, while they are easily photo- 
graphed. 

Per contra^ the ordinary photographic plate is by 
no means impartial — it is very sensitive to blue and 
purple, and very obtuse to green, yellow, and red. The 
new isochromatic and orthochromatic plates are, how- 
ever, better in this respect, and by using them (com- 
bined with a reasonable amount of patience) it is now 
possible to work down even to the red extremity of the 
visible spectrum. 

If the collimator of a spectroscope of any form be 
directed toward an ordinary lamp, or upon the incan- 
descent lime of a calcium-light, the observer will get 
simply a continuous spectrum ; a band of color shading 
gradually from the red to the violet, without markings 
or lines of any kind. If the instrument be turned to- 
ward the sun he will obtain something much more in- 




Largk Princeton Spectuoscope (Fitted for Piiotograpiiy). 



m 



TEE SPECTROSCOPE AND THE SOLAR SPECTRUM. 77 

teresting — a band of color, as before, but marked by 
hundreds and thousands of dark Hues, some fine and 
black, like hairs drawn across the spectrum, while others 
are hazy and indistinct. 

Most of them retain their appearance and position 
perfectly from day to day ; some of them, however, are 
more intense at one time than another, and when the sun 
is near the horizon certain lines in the red and yellow 
become extremely conspicuous, in such a way as to make 
it clear that they, at least, have something to do with our 

Fig. 22. 









I 

j 






f 

i 


1 
■ 






if 
11 1 






T 

1 


i 


IL. 

1 1 


1 


1 ; i 



A a B C 



E b 



terrestrial atmosphere. Fig. 22 is a reproduction of a 
portion of Fraunhofer's map of the solar spectrum, 
showing what one might fairly expect to see (except 
as to color) with an excellent single-prism spectroscope. 
Fig. 23 is a drawing of a very small portion of the 
spectrum in the green, as shown by a very powerful 
spectroscope. The scale is that of Angstrom's map. 
The large, heavy lines are known as the little h group, 
and are due, as we shall soon see, part of them to the 
presence of iron and nickel, and part to magnesium, as 
gases in the solar atmosphere. 

SPECTRUM MAPS. 

There are numerous maps of the solar spectrum : 
the earliest of any scientific value was Kirchhoif s, which 



Y8 



THE SUN. 



appeared in 1861-'62. Its scale was purely arbitrary, 
and not even self-consistent throughout, so that when 
Angstrom's map of the " normal spectrum " was pub- 



FiG. 23. 




b Grofp in Solar Spectrum. 

lished in 1868, made with a diffraction grating, and 
platted on a scale of wave-lengths (one unit of the scale 
corresponding to 07ie ten-millionth of a millirnetre of 
wave-length), it soon superseded Kirchhoff's, and has 
not yet ceased to be used for reference. Angstrom's 
grating was imperfect, however, and at present Row- 
land's photographic map, dating from about 1890, is 
accepted as " the standard." It covers the ultra-violet 
from about X^ 3000, and extends down through the 
visible spectrum into the red to X 6900, just below the 
B line. It is unfortunate that it does not go lower, but 
Mr. Higgs, of Liverpool, has published a set of photo- 
graphs of different parts of the spectrum, and two of 
these complete Rowland's map to the lower limit of 

* A, is the symbol now iinivcrsaUy used for the " wave-length " of a 
ray of light. " \ 3000 " means that part of the spectrum (invisible in 
this case) where the wave-length is 3000 ten-millionths of a millimetre. 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. Y9 

photographic capability. In clearness and beauty of 
execution Mr. Iliggs's maps surpass everything that 
has been done in that line. Thollon's great map cov- 
ers only the lower half of the spectrum, and is subject 
to the same objection as that of Kirchhoff — an arbitrary 
scale. Its peculiarity is in presenting the appearance 
of the spectrum corresponding to different altitudes of 
the sun. 

As there are invisible rays beyond the upper or 
violet end of the visible spectrum, so also below the 
red there is a still longer range of rays of wave-length 
too great for the human eye to perceive. Photography 
carries us a little way into this infra-red region, but not 
far. For most of our knowledge of it we are dependent 
upon the " bolometric " work of Professor Langley, to 
which we shall have to refer more specially in treating 
of the heat of the sun. He has already succeeded in 
mapping this " heat-spectrum," as it is sometimes called, 
sufficiently to show that it is filled w^ith dark bands and 
lines, and has fixed the position of many of them. 

If, instead of using the sun or an ordinary flame for 
the source of liglit, we examine with the spectroscope 
an electric spark, or the arc between carbon points, or 
the light produced by passing the discharge of an induc- 
tion coil througli a rarefied gas, we shall get a spectrum 
of quite a different sort — a spectrum consisting of bright 
lines upon a dark or faintly luminous background ; and 
it will be found that the spectrum developed will al- 
ways be the same under similar circumstances, depend- 
ing mainly upon the material of the electrodes (the 
points between which the discharge passes), and the 
nature of the intervening gas, but also, to a certain 
extent, upon its density and the intensity of the elec- 
tric discharge. So, also, if certain easily vaporized salts 



80 THE SUN. 

are introduced into the blue flame of a Bunsen gas-burn- 
er, or of a spirit-lamp even, the flame becomes colored.^ 
and its spectrum is a spectrum of bright lines, which 
are perfectly characteristic of the metal whose salt is 
used. An ordinary candle-flame, indeed, almost always 
shows one such bright line in the yellow, as had been 
noticed many years before Swan, in 1857, showed it to 
be due to the presence of sodium, which in the form 
of common salt is universally distributed. 

Fraunhofer, as early as 1814, had discovered that 
this line (or lines rather, for it is really composed of 
two, easily separated by a spectroscope of no great 
power) exactly coincides with the double line which he 
named D, in the solar spectrum ; and he had found the 
same line in the spectra of certain stars also ; but he did 
not know that the line was due to sodium, or in all 
probability he would have anticipated by nearly half a 
century the discovery which lies at the foundation of 
modern spectrum analysis. As has been said before, 
the principles involved seem to have been more or 
less distinctly apprehended by several persons — Stokes 
and Angstrom especially — years before the publication 
of Kirchhoff in 1859 ; but it was KirchhoS's work 
which first bore fruit. 

It is not necessary to repeat here again the oft-told 
story how he found that, when sunlight is made to 
pass through a flame containing sodium-vapor, the D- 
lines in the spectrum of this sunlight come out with 
increased intensity ; though, when a screen is interposed 
between the sun and the flame, the lines are bright, as 
usual in such a flame. He found, too, that when the 
incandescent lime-cylinder of the calcium-light is placed 
behind the sodium-flame, a precisely similar phenomenon 
occurs, and the bright lines of the flame-spectrum are 



4 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 81 

reversed to dark ones.^ He found the same thing to 
hold good also for a flame colored by lithium. 

The sum of his results may be stated as follows : 

1. Solids and liquids, when incandescent, give con- 
tinuous spectra ; and, as we now know, the same thing 
is true of gases also at great pressures. 

2. Bodies in the gaseous state (and not compressed) 
give discontinuous spectra consisting of bright lines and 
bands; and these bright-line spectra are diflFerent for 
different substances, and characteristic, so that a given 
substance is identitiable by its spectrum. 

3. When light from a solid or liquid incandescent 
body passes through a gas, the gas absorbs precisely 
those rays of which its own spectrum consists ; so that 
the result is a spectrum marked by hlack lines occupy- 
ing exactly the same positions which would be held by 
the bright lines in the spectrum of the gas alone. 

His conclusion, therefore, was that the luminous sur- 
face of the sun (the '^ photosphere ") is composed of 
solid or liquid matter giving by itself a purely contin- 
uous spectrum, and that the dark lines which mark the 
spectrum are produced by the transmission of the light 
through an overlying atmosphere. He believed the 
photosphere to be a continuous sheet of liquid — a mol- 
ten ocean — but numerous facts since learned make it 
almost certain that it is rather a sheet of '' cloud," com- 
posed of minute drops or dust floating in the lower re- 
gions of the solar atmosphere. 

* The blackness of the lines formed in this way is such that it is some- 
times difficult to believe, what is really the fact, that they are actually 
brighter than they were before the lime-cylinder was placed behind the 
flame, and that their darkness is only apparent, and due to their contrast 
with the more brilliant background of the continuous spectrum of the 
incandescent lime. It is very easy, however, to demonstrate the truth by 
a simple experiment. 
7 



82 THE SUN. 

If, then, sodium is present in the solar atmosphere 
between us and the photosphere, we ought to find in 
the solar spectrum those lines dark which are bright in 
the spectrum of sodium-vapor ; and we do. If mag- 
nesium is there, it ought to manifest itself in the same 
way, and it does ; and similarly for all the substances 
which spectrum analysis reveals. 

If this view is correct, it follows also that this atmos- 
phere, containing in gaseous form the substances whose 
presence is manifested by the dark lines of the ordi- 
nary spectrum — the sun's reversing layer^ as it is now 
often called — would give a spectrum of bright lines if 
we could isolate its light from that of the photosphere. 
The observation is possible only under peculiar circum- 
stances. At a total eclipse of the sun, at the moment 
when the advancing moon has just covered the sun's 
disk, the solar atmosphere of course projects somewhat 
at the point where the last ray of sunlight has disap- 
peared. If the spectroscope be then adjusted with its 
slit tangent to the sun's image at the point of contact, a 
most beautiful phenomenon is seen. As the moon ad- 
vances, making narrower and narrower the remaining 
sickle of the solar disk, the dark lines of the spectrum 
for the most part remain sensibly unchanged, though 
becoming somewhat more intense. A few, however, 
begin to fade out, and some even turn palely bright a 
minute or two before the totality begins. But the mo- 
ment the sun is hidden, through the whole length of 
the spectrum, in the red, the green, the violet, the 
bright lines flash out by hundreds and thousands, almost 
startlingly ; as suddenly as stars from a bursting rocket- 
head, and as evanescent, for the whole thing is over 
within two or three seconds. The layer seems to be 
only something under a thousand miles in thickness, 
and. the moon's motion covers it very quickly. 



i 



THE SPECTROSCOPE AND THE SOLAPv SPECTRUM. S3 

The phenomenon, though looked foi* at the first 
ecKpses after solar spectroscopy began to be a science, 
was missed in 1868 and 1869, as the requisite adjust- 
ments are delicate, and was first actually observed only 
in 1870. Since then it has been more or less perfectly 
seen at every eclipse. Except at an eclipse it has not 
yet been found possible to observe this bright-line spec- 
trum, because it is overpowered by the aerial illumina- 
tion of our own atmosphere. 

It is not, however, to be understood that the dark 
lines of the solar spectrum are due entirely or even 
principally to the stratum of gas which lies close above 
the surface of the photosphere. "Were this so, the 
dark lines should be much stronger in the spectrum of 
light from the edges of the disk than in that from the 
center, which is not the case ; at least, the difference is 
very slight. The photosphere, as we shall see here- 
after, is probably composed of separate cloud-like masses 
floating in an atmosphere containing the vapors by 
whose condensation they are formed ; the principal ab- 
sorption, therefore, probably takes place in the inter- 
stices between the clouds, and below the general level 
of their upper limit. 

The beautiful observations of Professor Hastings, 
of Baltimore, in which by an ingenious contrivance he 
managed to confront and compare directly the spectra 
of light from the center and edges of the sun's disk, 
have brought out the facts in the case very finely. 

Theoretically, then, it is very easy to test the ques- 
tion of the presence of an element in the sun. It is 
only necessary to cover one half the length of the spec- 
troscope-slit with a mirror or prism by which the sun- 
light is directed into the instrument, while at the same 
time a flame or electric spark, giving the spectrum of 
the substance under investigation, is placed directly in 



84 



THE SUN. 



front of the other half of the slit. When matters arai 
thus arranged, the observer sees in tlie instrument twa 
spectra in juxtaposition, each of half the usual width 
one the solar spectrum, the other that of the element 
under investigation ; and it is easy to see whether the 
bright lines of the elementary vapor match exactly 
with corresponding dark lines in the solar spectrum. 




Fig. 25, 




Action of the Comparison- 
Prism. 



Comparison Prism at the Slit of the Spec- 
troscope. 



The figures show the usual arrangement of the com- 
parison-prism, as it is ordinarily called. 

For the examination of the upper or violet portion 
of the spectrum, photography is employed with great 
advantage, the arrangement being precisely the same as 
that just indicated, except that a sensitized plate takes 
the place of the human retina, and the impression can 
be permanently retained for leisurely study. Certain 
light, too, as every one knows, which is invisible to the 
eye, strongly affects the photographic plate, so that the 
comparison can by this means be carried on into the 
ultra-violet regions of the spectrum. 

The following full-page illustration is a representa- 
tion of the arrangement of apparatus used by Mr. Lock- 
yer in his celebrated researches — it is taken from his 
" Studies in Spectrum Analysis." 



THE SPECTROSCOPE AND THE SOLAP. SPECTPvUM. 85 




86 THE SUN. 

Theoretically, we say, the comparison is easy ; but 
the practical difficulties are considerable. In the first 
place, it is not easy to get a spectrum of the body you 
wish to study, free from lines belonging to other sub- 
stances — the requisite chemical purity is very trouble- 
some to attain ; and, in the next place, the dark lines of 
the solar spectrum are so numerous that it requires a 
very high dispersive power to establish a coincidence 
with certainty ; a bright line in the spark-spectrum may 
fall very near a dark line with which it has no connec- 
tion w^hatever. When, however, as in the case w^e have 
mentioned, the coincidences are not one or tw^o, but 
numerous, and the lines in question peculiar in their 
character and appearance, a satisfactory result is soon 
established. 

It was in this manner (by comparisons made by the 
eye and not by photography) that Kirchhoff in 1860 de- 
termined the presence in the solar atmosphere of the 
following elements : sodium, iron, calcium, magnesium, 
nickel, barium, copper, and zinc, the last two rather 
doubtful at that time. Since then the list has been 
greatly extended, and in 1891 stood as follows, accord- 
ing to Professor Rowland, who has been making a most 
thorough reinvestigation of the whole subject. He has 
worked with concave-grating apparatus of the highest 
power, and has made his comparisons between the spec- 
trum of the sun and the spectra of the chemical ele- 
ments by means of photography. He has used, however, 
only the electric arc^ and not the spark, in producing 
tlie spectra, and it is to be hoped that this research 
may be supplemented by an equally thorough study of 
spark-spectra. Since the work is not even yet com- 
plete (1895), the list is to be regarded as provisional 
only. 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. §7 



ELEMENTS IN THE SUN, ARRANGED ACCORDING TO THE 

NUMBER AND INTENSITY OF THEIR DARK 

LINES IN THE SOLAR SPECTRUM. 





Intensity. 


Number. 


1. 


Calcium. 


Iron (2,000 or more). 


2. 


Iron 


Nickel. 


o. 


Hydrogen. 


Titanium. 


4. 


Sodium. 


Manganese. 


5. 


Nickel. 


Chromium. 


6. 


Maf^nesium. 


Cobalt. 


7. 


Cobalt. 


Carbon (200 or more). 


8. 


Silicon.f 


Vanadium. 


9. 


Aluminium, f 


Zirconium. 


10. 


Titanium. 


Cerium. 


n. 


Chromium. 


Calcium (75 or more). 


12. 


Strontium. 


Ncodymium. 


13. 


Manganese. 


Scandium. 


14. 


Vanadium. 


Lanthanum. 


15. 


Barium. 


Yttrium. 


16. 


Carbon.f ? 


Niobium. 


17. 


Scandium, f 


Molybdenum. 


18. 


Yttrium. 


Palladium. 


19. 


Zirconium, f 


Magnesium (20 or more). 


20. 


Molybdenum, f 


Sodium (11). 


21. 


Lanthanum. 


Silicon. 


22. 


Niobium, f 


Hydrogen. 


23. 


Palladium.! 


Strontium. 


24. 


Neodymium.f ? 


Barium. 


25. 


Copper, f 


Aluminium (4). 


26. 


Zinc. 


Cadmium. 


27. 


Cadmium. 


Rhodium. 


28. 


Cerium. 


Erbium. 


29. 


Glucinum.f 


Zinc. 


30. 


Germanium, f 


Copper (2). 


31. 


Rhodium, f 


Silver. 


32. 


Silver. 


Glucinum. 


33. 


Tin. 


Germanium. 


34. 


Lead. 


Tin. 


35. 


Erbium. 


Lead (1). 


36. 


Potassium, f 


Potassium. 



88 


THE SUN. 
DOUBTFUL ELEMENTS. 




Iridium. 


Osmium. Platinum. 


Ruthenium. 


Tantalum. 


Thorium. Tungsten. 


Uranium. 



ELEMENTS NOT APPEARING IN THE SOLAR SPECTRUM. 

Antimony. Arsenic. Bismuth. Boron. 

Caesium. Gold. Indium. Lithium. 

Phosphorus. Rubidium. Selenium. Mercury. 

Thallium. Prseseodymium. Nitrogen (vacuum-tube). Sulphur. 

SUBSTANCES NOT YET TRIED (bY ROWLANd). 

Bromine. Chlorine. Fluorine. Iodine. 

Oxygen. Gallium. Holmium. Tellurium. 

Terbium. Thulium, etc. 

Professor Rowland remarks that several of the ele- 
ments are classified as " not appearing in the solar spec- 
trum " merely because their arc-spectra show very few 
strong lines, or none at all, within the limits of the solar 
spectrum (it might be different w^ith their spark-sipec- 
tra). 

He adds, what can not too firmly be borne in mind, 
that the failure to find them " is very little evidence of 
their absence from the sun itself," and that if the whole 
earth " were heated to the temperature of the sun, its 
spectrum w^ould probably resemble that of the sun very 
closely." 

Besides these substances which reveal their presence 
in the sun by dark lines in its spectrum, there are at 
least two others, helium and coronium, as they are 
provisionally named, which show themselves only by 
bright lines in the spectrum of the chromosphere and 
the corona, with which we shall deal later. In 1895 
helium was at last identified by Ramsay, in connection 
with his researches upon argon, the new component of 
our atmosphere. He found the lines of helium in gas 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 89 

disengaged from uraninite and otlier minerals, where 
it is associated with the so-called " rare earths." Coro- 
ninm still remains undetermined. 

All of the above-named elements, except those 
marked with a +, are represented at times by bright lines 
in the spectrum of the chromosphere, which will be dis- 
cussed in another chapter ; and strontium and cerium 
were observed in that manner by the writer before the 
coincidence of their lines wuth dark lines in the ordinary 
solar spectrum had been satisfactorily made out. 

As to carbon, the characteristic groups of lines 
which mark the visible portion of its spectrum are only 
doubtfully present ; but in the ultra-violet the photo- 
graphs of Mr. Lockyer have brought out other groups 
which belong to this element, and the presence of this 
element has since been abundantly confirmed by Row- 
land and others. 

Thus far the most careful observation fails to find, 
either in the ordinary spectrum or in that of the chro- 
mosphere, the slightest trace of bromine, chlorine, 
iodine, or nitrogen, of arsenic, boron, or phosphorus ; 
of sulphur there are merely doubtful indications in the 
chromosphere spectrum ; and as regards oxygen, the 
evidence, on the whole, is against its presence, though 
the case is peculiar. 

When we recollect that the non-apparent elements 
constitute a great portion of the earth's crust, the 
question at once forces itself, What is the meaning of 
their seeming absence ? Do they really not exist on 
the sun, or do they simply fail to show themselves ; and, 
if so, why ? The answer to the question is not easy, 
and astronomers are not agreed upon it, though we im- 
agine that most of them would prefer the latter alterna- 
tive. Even under the conditions of our terrestrial 



90 THE SUN. 

laboratories we find cases where, when several gases and 
vapors are mingled at a high temperature, certain ones 
only of those present appear in the spectrum of the 
mixture, the others giving no indication of their pres- 
ence. Then, too, it is now certain that the same sub- 
stance under differing conditions may give two or more 
widely differing spectra ; it is easy to admit, therefore, 
that we may be unable to reproduce in the electric arc 
tlie spectrum of a substance that characterizes it in 
the sun, and so may fail to identify it. Possibly, in 
some cases, the very brilliance of the lines of an element 
may prevent their appearance as dark lines. It is pos- 
sible, for instance, to make the bright lines of sodium 
so intense that the light from an incandescent lime- 
cylinder will not be able to reverse them, and, of course, 
by making them a little less intense, they may be caused 
to disappear entirely, being neither brighter nor darker 
than the continuous spectrum on which they are pro- 
jected. This may perhaps actually be the case with 
helium, which gives in the chromosphere spectrum an 
intensely brilliant yellow line, known as D3, because it 
is very near to the sodium lines, D and D,. At times, 
and especially in the neighborhood of sun-spots, a very 
faint dark line marks its place, but usually the spec- 
trum of the photosphere fails to give the slightest in- 
dication of its presence. There are, however, in the 
chromosphere spectrum fifteen or twenty other bright 
lines (but fainter than D3) which have no dark cor- 
relatives. Most of these are now known to be also 
due to helium, and this makes it more likely that the 
absence of dark lines is accounted for either by the 
thinness of the helium layer or the intensity of its tem- 
perature. 

Xitrogcn and hydrogen each have two spectra, one 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 91 

a spectrum mostly composed of shaded bands, while the 
other consists of sharp, well-defined lines. Oxygen, ac- 
cording to Schuster's careful researches, has four spec- 
tra, and carbon is also assigned four by its investigators. 
There appear to be various possible explanations of 
these facts. One is, to suppose that the luminous sub- 
stance, without any change in its own constitution, vi- 
brates differently and emits different rays under varying 
circumstances, just as a metal plate emits various notes 
according to the manner in which it is held and struck. 
A second assumes that the substance, without losing its 
chemical identity, undergoes changes of molecular struct- 
ure (assumes allotropic forms) under the varying cir- 
cumstances which produce the changes in its spectrum. 
According to either of these views, although we can 
safely infer, from the presence of the known lines of an 
element in the solar spectrum, its presence in the solar 
atmosphere, we can not legitimately draw any negative 
conclusion : the substance may be present, but in such 
a state under the solar conditions as to give a spectrum 
different from any with which we are acquainted. 

Still a different explanation is to suppose, with Mr. 
Lockyer, that the changes in the spectrum of a body are 
indications of its decomposition, the spectrum of tlie 
original substance being replaced by the superposed 
spectra of its constituents, so that the absence of the 
missing substances is real, being due simply to the fact 
that the solar atmosphere is too hot to permit them to 
exist in it: they decompose or "dissociate" at a lower 
temperature. 

It would be improper to dismiss this hypothesis with 
a mere passing mention, for during the past fifteen years 
it has been almost constantly under brisk discussion. 
Whether true or not, it is certainly not absurd, nor in 



92 THE SUN. 

itself even improbable. The idea that at bottom there 
is but one material substance, and that all our chemical 
elements differ only in the way in which their ultimate 
molecules are built up out of the simple atoms of this 
" pantogen," is old, and has always been attractive to 
speculative minds. Grant it, and many otherwise puz- 
zling facts and relations of the new chemistry become in- 
telligible. At the same time it has not yet been proved, 
and so far all attempts to break up the elements into 
simpler bodies have failed. It seems impossible also to 
reconcile the hypothesis with the laws which connect 
the specific heat of bodies with their chemical constitu- 
tion and atomic weight. 

It may be added, too, that some of the supposed ob- 
servational facts upon which Mr. Lockyer relied at 
first to support his theory have turned out to be mis- 
takes due to errors of experimentation, or the use of 
insufficient spectroscopic power. 

Thus great stress was laid upon the so-called " hasic " 
lines which appear to be common to the spectra of dif- 
ferent substances. If one runs over Angstrom's map 
of the solar spectrum he will find about twenty-five lines 
marked as belonging both to iron and calcium. The 
same is true of iron and titanium to a still greater 
extent, and to a considerable degree of several other 
pairs of substances. This fact might be explained in 
several ways. The common lines may be due^first, to 
impurities in the materials worked with ; or, second, to 
some common constituent in the substances (which is 
Lockyer's view) ; or, third, to some similarity of molec- 
ular mass or structure which determines an identical 
vibration-period for the two substances ; or, finally, it 
may be that the supposed coincidence of the lines is 
only apparent and approximate — not real and exact — 



THE SPECTROSCOPE AND TUE SOLAR SPECTRUM. 93 

in which case a spectroscope of sufficient dispersive 
power would show the want of coincidence. 

Now, Mr. Lockyer, by a series of most laborious 
researches, has proved that many of the coincidences 
shown on the map are merely due to impurities, and 
he has been able to point out which of the lines mapped 
as common to calcium and iron, for inst ce, belonged 
to each metal. As the iron employed is rendered suc- 
cessively purer and purer, certain of the common lines 
become fainter, and such evidently belong to calcium 
and not to iron. Similarly, when calcium is used, we 
can point out the lines which are due to the iron con- 
tamination. But, when all is done, we find that certain 
of the common lines persist, becoming more and more 
conspicuous with every added precaution taken to in- 
sure purity of materials. 

Moreover, w^hen one of the substances, say the cal- 
cium, is subjected to continually increasing tempera- 
tures, its spectrum is continually modified, and these 
hasic-Unes^ as Mr. Lockyer asserts, are the ones which 
become increasingly conspicuous, while others disappear. 
This is just what ought to happen if they are due to 
some element existing in both the iron and calcium — an 
element liberated in increasing abundance with every 
rise of temperature. 

But unfortunately for the theory the application of 
our present powerful spectroscopes shows that in almost 
every case these '' basic " lines are only instances of 
close coincidence. The writer in 1880 examined with 
care the seventy lines given on Angstrom's map as com- 
mon to two or more elements, and was able to resolve 
fifty-six of them into doubles or triplets ; and later ob- 
servers have resolved the rest or shown that they were 
due to impurities. Professor Kowland remarks that 



94 THE SUN. 

" with the high dispersion " employed by him " Lock- 
yer's ^basic-lines' are widely broken up and cease to 
exist." 

As has been already remarked, the case of oxygen is 
peculiar. The great A and B bands of the solar spec- 
trum are certainly due to this gas, as Egoroff first defi- 
nitely proved in 1883 ; but, as the experiments and ob- 
servations of Janssen and others have since abundantly 
demonstrated, it is the oxygen of the eariKs atmosphere 
and not oxygen in the sun that produces them ; they 
belong to a low-temperature spectrum of the gas, and 
not to the spectrum produced by the electric arc or 
spark. 

But in 1877 the late Dr. Henry Draper, of New 
York, announced that he had discovered the presence of 
oxygen in the sun, and he published photographs which 
show, in a very plausible manner, the coincidence be- 
tween the bright lines of this element and certain hright 
spaces or bands in the solar spectrum. His method of 
precedure was to form the spectrum of oxygen by 
means of sparks from a powerful induction-coil, w^orked 
by a dynamo-electric machine, itself driven by an en- 
gine. These sparks passed between iron terminals, in 
a little chamber wrought out of soapstone, through w^iich 
a current of pure oxygen was forced at atmospheric 
pressure nearly ; sometimes, however, air was used in- 
stead, giving the same results, except that the spectrum 
of nitrogen was then superadded to that of oxygen. 
The spectrum of this spark w^as photographed simultane- 
ously with that of the sun, the sunlight being brought 
in through half the slit by a small reflector, and thus a 
comparison was obtained, free from personal bias, be- 
tween the solar spectrum and that of the gas. The iron 
lines, due to the terminals, are a great assistance in test- 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 95 

ing the adjustments. The oxygen lines produced in this 
way at atmospheric pressure are not so well defined as 
those seen in the spectrum of a Geissler tube, but are 
rather broad and hazy. 

In tlie blue portion of the solar spectrum, which 
alone is accessible to photography, the Fraunhofer lines 
are generally very numerous, close, and black ; but here 
and there is an interval free, or comparatively free, 
from lines. In a low-dispersion spectroscope such an 
interval looks like a bright band. Now, almost every 
one of the dozen or so bright lines of oxygen, w^hich 
the photographs display, falls exactly against one of 
these brighter interspaces, and it seems hardly probable 
that so many coincidences can be merely due to chance. 

Dr. Draper afterward repeated the laborious and 
expensive experiments in a still more elaborate manner, 
and w^th results entirely confirmatory. 

It is, however, extremely diflicult to explain how 
oxygen in the sun's atmosphere can produce such an 
eft'ect in the ordinary solar spectrum while remaining 
invisible in the spectrum of the chromosphere ; and the 
most careful search does not show in it a single one of 
these bright oxygen-lines. We say of ihese\\\i^^^ because 
Dr. Schuster has shown, with great probability, that a 
different oxygen spectrum, with only four bright lines 
in it, has these four all represented by dark lines in the 
photospheric spectrum, and two of the four in the spec- 
trum of the chromosphere. 

With high dispersive powers, the " bright bands " 
of the solar spectrum entirely lose their prominence, and 
are even found to be occupied by numerous fine dark 
lines. Dr. John C. Draper has suggested that these 
dark lines may be the true representatives of oxygen. 

Still later photographic comparisons between the 



96 THE SUN. 

solar spectrum and that of oxygen, made with high dis- 
persive powers, in the Physical Laboratory of Harvard 
College and at other places, render it certain that no 
coincidences of solar-lines with the bright lines in the 
line'Sjpectrurri of oxygen exist in the region covered by 
the photographic plates that were used (X 3,750 to 
\ 5,034). Probably most spectroscopists consider this 
conclusive as to the absence of oxygen from the solar 
spectrum : at the same time, as Professor Pickering has 
shrewdly said, ^' one would scarcely expect to recognize 
a friend's countenance under the rriicTOSGOjper The 
discussion can hardly be considered as finally closed. 

The lines of the solar spectrum not only inform us 
as to the presence or" absence of bodies in the solar 
atmosphere, but give us, to some extent, indications as 
to their physical condition. The spectrum of a given 
body, say hydrogen, varies very much in the relative 
strength and brightness of its lines, according to the 
circumstances of its production. If, for instance, the 
gas be highly rarefied, and the electric spark, which 
illuminates it, not too strong, the lines will be fine and 
sharp. Under higher pressure and more intense dis- 
charges, some of them will become broad and hazy, and 
new lines, before unseen, will make their appearance. 
So of other substances ; and this apart from the fact, 
before stated, that a given element often has several en- 
tirely different spectra. Changes, such as have been 
mentioned, go on up to a certain point, and then, sud- 
denly, an entirely new spectrum appears, not having 
apparently the slightest connection with the one which 
preceded it any more than if it came from an entirely 
different element or mixture of elements ; as, in fact, 
according to Mr. Lockyer's view, is probably the case. 

Now, in the solar spectrum, the dark lines character- 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 97 

istic of an element are all coincident with bright lines 
of its gaseous spectrum ; but it is often the case that 
the relative width and intensity of the solar-lines do not 
match those of the bright lines in the spectrum ob- 
tained by artificial means. In the spectrum of calcium, 
for instance, certain lines, which in our laboratory ex- 
periments are the most conspicuous, are very faint upon 
the sun, and others, which are inconspicuous in the spark 
spectrum, are vastly more important on the solar sur- 
face. As yet, w^e are not able wdth certainty to inter- 
pret all these variations, but, in a general way, it may be 
said that they all point to the conclusion that the tem- 
perature of the solar atmosphere is considerably higher 
than that of any of our flames or electric arcs. 

SPECTROSCOPIC INDICATIONS OF MOTION. 

At times, also, when the motions of the solar atmos- 
phere become unusually intense, the spectroscope ap- 
prises us of the fact, and gives us the means of deter- 
mining the rate at which the moving masses are advanc- 
ing toward us or receding from us. If a luminous 
body is approaching with a velocity at all comparable 
with that of light, the pitch of the light, if the expres- 
sion may be allowed — its wave-length and number of 
vibrations per second — will be changed and heightened 
just as in the case of sound. 

Most of our readers have probably noticed the curi- 
ous change in pitch of the bell or Avhistle of a locomo- 
tive passing at full speed, especially if we ourselves 
were on a train moving in the opposite direction. If 
the velocity is great (about forty miles an hour for each 
of the trains) the pitch will drop a full third. 

The explanation, first given by Doppler, of Prague, 
in 1842, is simply this : If both we and the locomotive 



98 THE SUK 






carrying the bell were at rest, we should hear the bell's 
true sound, the pulsations following each other at regu- " 
lar and the real intervals. If, now, we are rapidly ap- 
proaching the bell, the interval of time between the 
impact of each pulse upon the ear and the following 
one will be shortened, because after any pulse has been 
received we advance part way to meet the next, and so 
encounter it earlier than if we had remained at rest. 
Now, this interval of time between successive pulsations 
is precisely what determines the pitch of the sound : the 
more pulsations there are in a second the higher the 
pitch. It is obvious that, if we remain at rest and the 
bell approaches us, the same effect will be produced, 
and that, if both are moving, the effects will be added ; 
and, finally, it is clear that the recession of the hearer 
from the bell will produce the opposite effect and lower 
its pitch. 

Just the same thing holds good of light ; it also con- 
sists of pulsations, and the refrangibility of a ray and 
its diffrangiMlity^ if we may coin the word, both de- 
pend upon the number of pulsations per second Avith 
which it reaches the diffracting or refracting surface. 
The more frequent the pulsations the more it will be 
refracted, and the less it will be diffracted. If, then, 
we were swiftly approaching a mass, say of incandes- 
cent hydrogen, we should find the position of each ofl 
its characteristic rays in the spectrum slightly altered," 
and falling farther from the red end of the spectrum 
(the region of slow vibrations) than if we were at rest. 
By comparing the positions of these lines with thos' 
obtained from a Geissler tube containing hydrogen, w< 
could find how much change was produced, and there- 
fore how the velocity with which we are approaching! 
the moving mass compares with that of light. Simi 



THE SPECTROSCOPE AND THE SOLAR SPECTRUM. 99 

larly, if the body were advancing toward us. And, vice 
versa^ if the distance were increasing, the lines would 
be shifted downward in the spectrum toward the red."^ 

Because the velocity of light is exceedingly great 
(more than 186,000 miles per second), it is evident that 
only very swift motions can produce any sensible dis- 
placement of lines in the spectrum. Since, however, 
in the neighborhood of sun-spots and in the solar prom- 
inences, we frequently meet with masses of gas moving 
from thirty to fifty miles a second, and sometimes as 
much as three hundred miles a second, it is not unusual, 
in working with the telespectroscope, to observe the dis- 
tortion and displacement of portions of a dark line 
vhicli are produced by these motions, and indicate 
them. 

The figure represents the appearance of the C line 
seen in the spectrum of a sun-spot by the writer on 
Sept. 22, 1870. The velocities indicated vary from two 
hundred and thirty to three hundred and twenty miles 
per second ; the latter is seldom, if ever, exceeded. 

Results of this sort are so surprising that there have 
been many attempts to escape from them, and to ac- 
count for the distortion of lines in some other way, but 

* The formula for computing the change of wave-length produced by a 
given velocity along the line of sight is very simple. Let \ be the real wave- 
length of the ray ; A', the apparent wave-length as affected by the motion ; 
F, the velocity of light (186,330 miles a second) ; and v, the rate at which 
the distance between the observer and the source of light is increasing, 
then A — A = A—, which may be written A A = a—. If the distance is 

decreasing^ v must be taken as minus, and A' will be less than A. 

As an example, suppose that near a sun-spot a mass of hydrogen is 
approaching us at the rate of 50 miles a second : how much will the wave- 
length of the C line (A = 6,563 units) be diminished ? 

A A = 0,563 X TirMoo = ff S = I*'?? units. That is, the C line will 
be shifted up (toward the blue) I'll units on the scale of Rowland's map. 



100 



THE SUN. 



without any satisfactory success. There have been diffi- 
culties raised also in regard to the mathematical theory 
of the matter. These have been met, however ; and 
what amounts to an experimental verification of the 
correctness of the received view has been reached by 
measurements of the displacement of lines in the spec- 
tra of the eastern and western limbs of the sun. The 
eastern limb is moving toward us, the western from us, 




Changes in thh C Line (September 22, 18T0). 

in consequence of the sun's rotation, each with a ve- 
locity of about 1'16 miles per second. The resulting 
displacement of the lines is, of course, very slight — only 
about Y^Q of the distance between the two D lines — 
but, small as it is, it has been satisfactorily detected and 
measured by several observers — Zollner, Yogel, Lang- 
ley, and the writer, among the earlier. 

The values determined have ranged generally some- 
what larger than 1'16. My own result was 142 ± 0'07, 
and was obtained in 1876 with the first grating-spectro- 
scope used in astronomical work. 

A later determination made by Crew, at Baltimore, 
with a much more powerful instrument, gave 1*18. The 






THE SPECTROSCOPE AND THE SOLAR SPECTRUM. IQl 

most complete and satisfactory series of observations of 
this sort is, however, that made by Duner in 1887-'S9, 
which not only gave a good determination of the 
sun's rotation period (25*56 days), agreeing closely with 
that deduced from spot observations, but also brought 
out clearly the " equatorial acceleration " (page 140). 

Cornu has made a beautiful application of this prin- 
ciple to enable one to discriminate immediately between 
the lines in the solar spectrum which are really " solar " 
and the " telluric " lines, which are due to our own at- 
mosphere. A small image of the sun is formed upon 
the slit-plate of the spectroscope by a lens which can be 
made to swing back and forth three or four times a 
second. This makes the solar image oscillate across the 
slit-plate in an east and west direction, and to the ob- 
server all the true solar lines appear to quiver, while 
the telluric lines stand fast. 

In the motion-distortions of lines Lockyer finds 
strong confirmation of his ideas. It not unfrequently 
happens that in the neighborhood of a spot certain of 
the lines which we recognize as belonging to the spec- 
trum of iron give evidence of violent motion, while, 
close to them, other lines, equally characteristic of the 
laboratory spectrum of iron, show no disturbance at all. 
If we admit that what we call the spectrum of iron is 
really formed in our experiments by the superposition 
of two or more spectra belonging to its constituents, 
and that on the sun these constituents are for the most 
part restricted to different regions of widely varying 
pressure, temperature, and elevation, it becomes easy to 
see how one set of the lines may be affected without the 
other. 

But the same facts are also explicable on various 
other hypotheses. 



CHAPTEE lY. 

su:^^-sP0TS and the solar surface. 

Granulation of Solar Surface. — Views of Langley, Nasmith, Secchi, and 
others. — Faculse. — Nature of the Photosphere. — Janssen's Photo- 
graphs of Solar Surface — the Reseau Photospherique. — Discovery of 
Sun-Spots. — General Appearance and Structure of a Spot. — Its For- 
mation and Disappearance. — Duration of Sun-Spots. — Eemarkable 
Phenomena observed by Carrington and Hodgson. — Observations of 
Peters. — Dimensions of Spots.— Proof that Spots are Cavities. — Sun- 
Spot Spectrum. — " Veiled Spots." — Rotation of Sun. — Equatorial Ac- 
celeration. — Explanations of the Acceleration. — Position of Sun's 
Axis and Sccchi's Table for its Position- Angle at Different Times of 
the Year. — Proper Motions of Spots. — Distribution of Spots. 

When an observer, provided with suitable telescopic 
appliances, examines the surface of the sun, he finds a 
most interesting field before him. At first view, in- 
deed, it is less impressive than the moon ; there is not 
so much to attract the immediate attention — no moun- 
tain-ranges and craters, no shadows, rills, or rays. 

But, if the telescope is a good one and the atmos- 
pheric conditions favorable, the details soon begin to 
come out : the surface is seen to be far from uniform, 
composed of minute grains of intense brilliance and ir- 
regular form, floating in a darker medium, and arranged 
in streaks and groups. If the magnifying power em- 
ployed is rather low, the general effect of the surface is 
much like that of rough drawing-paper, or of curdled 
milk seen from a little distance ; and, generally speak- 
ing, a low power is all that can be used, because the 



SUN-SPOTS AND THE SOLAR SURFACE. 103 

lieat of the sun commonly keeps tlie air in a state of great 
disturbance, so that it is only occasionally that the solar 
surface can be scrutinized with such powers as we con- 
tinually employ upon the moon and planets. But now 
and then times come — favorable minutes, and even hours 
— when the telescopic power can be pushed to its maxi- 
mum, and we get such views as that w^hich Professor 
Langley has presented in the beautiful drawing of which 
our frontispiece is a reproduction. The grains, or " nod- 
ules," as Herschel called them, are then seen to be ir- 
regularly rounded masses, measuring some hundreds of 
miles each way, sprinkled upon a less brilliant back- 
ground, and making much the same impression as snow- 
flakes sparsely scattered over a grayish cloth, to use the 
comparison of Professor Langley. If the telescope has 
a diameter of not less than nine inches, and if the see- 
ing is absolutely exquisite, then these grains themselves 
are sometimes resolved into "granules," little luminous 
dots not more than a hundred miles or so in diameter, 
which, by their aggregation, make up the grains, just as 
they in their turn make up the coarser masses of the 
solar surface. Professor Langley estimates these gran- 
ules to constitute perhaps about one fifth of the surface 
of the sun, while they emit at least three quarters of the 
light. He and Secchi seem to be so far the only ob- 
servers wiio have ever fairly seen them. The " grains " 
have been known for years and described by many 
observers, but w^ith some very embarrassing discrep- 
ancies. Nasmyth, in 1861, described them as "wil- 
low-leaves " in shape, several thousand miles in length, 
but narrow, wdth pointed ends ; and figured the surface 
of the sun as a sort of basket-work formed by the inter- 
weaving of such filaments. Fig. 28 is copied from one 
of his pictures. His statement excited a good deal of 



104 



THE SUN. 



¥ia. 28. 



^msm^mmmmmmmm 




Group of Solar Spots observed and drawn by Nasmtth (June 5, 1864). 



4 



SUX-SPOTS AND THE SOLAR SURFACE. 



105 



pretty warm discussion. Dawes entirely denied the 
existence of any such forms ; while Stone and Secchi 
assigned them much smaller dimensions, and compared 
Huggins agreed completely with 



them to rice-grains. 



Fig. 29. 




Granttles and Poees op the Sun^s Surface. (After Ha^ins.) 

neither, but represents the '' make-up " of the solar sur- 
face by a drawing from which Fig. 29 is taken. This, 
is unquestionably a very correct delineation of what is 
seen with a good telescope under circumstances fair, 
but not the best possible. 



106 THE SUN. 

On portions of tlie sun's disk, however, the element- 
ary structure is often composed of long, narrow, blunt- 
ended filaments, not so much like " willow-leaves " as 
like bits of straw, lying roughly parallel to each other 
— a ''thatch-straw" formation, as it has been called. 
This is specially common in the penumbrse of spots, or 
in their immediate neighborhood. 

If one were to speculate as to the explanation of the 
grains and thatch-straws, it might be that the grains are 
the upper ends of long filaments of luminous cloud, 
which, over most of the sun's surface, stand approxi- 
mately vertical, but in the penumbra of a spot are in- 
clined so as to lie nearly horizontal. This is not certain, 
though ; it may be that the cloud-masses over the more 
quiet portions of the solar surface are really, as they 
seem, nearly globular, while near the spots they are 
drawn out into filamentary forms by atmospheric cur- 
rentSo 

Whatever the explanation may be, the appearance 
of things in the immediate neighborhood of a spot is 
often pretty fairly represented by Mr. Nasmyth's pict- 
ures, though that of Professor Langley is decidedly 
more accurate in details, and represents far better see- 
ings. 

Near the edges of the disk the light falls off very 
rapidly, and certain peculiar formations, called the fac- 
ulse, are there much more noticeable than near the cen- 
ter of the disk. These f aculae (Latin, " a little torch ") 
are irregular streaks of greater brightness than the gen- 
eral surface, looking much like the flecks of foam which 
•mark the surface of a stream below a waterfall. ISTot 
unfrequently they are from five to twenty thousand 
miles in length, covering areas immensely larger than 
any terrestrial continent. 



SUN-SPOTS AND THE SOLAR SURFACE. 107 

The figure, taken from a photograph by De La Rue, 
gives a reasonably correct idea of the general appear- 
ance of these objects, and of the darkening at the limb 
of the sun. No woodcut, however, is quite competent 
to give the delicate flocculence of the details. 

Until lately these faculge have been considered to be 
simply elevated portions of the photosphere — mountain- 
ous billows of shining cloud which rise above the gen- 

FiG. 30. 




_ _ _ ' V.-VV 

Sun-Spots and Facul^. (From a Photograph.) 

eral level, and protrude through the denser portions of 
the solar atmosphere. Occasionally, when one of them 
passes over the edge of the disk, it can be seen to pro- 
ject like a little tooth — the reader should not forget, 
however, that the elevation, to be perceptible at all, 
must be at least two hundred and twenty-five miles, or 
some forty-five times as high as any Himalaya. 

If they are elevations rising from the photosphere, 
the reason why they are so much more conspicuous 



108 



THE SUN. 



near the limb is simply this : The luminous surface is 
covered, as has been intimated before, with an atmof^- 
phere which is not very thick compared with the di- 
mensions of the sun, but still sufficient to absorb a good 
deal of the light. Light coming from the center of the 
disk penetrates this atmosphere at a, as is apparent from 
the figure, under the most favorable conditions, and is 
but slightly reduced in amount. The edges of the disk, 
on the other hand, are seen through a much greater 
thickness of atmosphere, as at J, and are, therefore, of 
course, much obscured, the amount of absorption being 



Fig. 31. 




by some observers put as high as seventy five per cent. 
If, now, to take an extreme case, we suppose a facula 
high enough to lift its summit quite through this at- 
mosphere, it will itself suffer no diminution of brill- 
iance while the sun's rotation carries it from the center 
of the disk to the limb, but it will have passed from 
a background of brightness almost equal to its own, 
on which it would be seen only with difficulty, to 
another seventy-five per cent, or so darker, and will 
thus become very conspicuous. What is true of faculse ' 
of such extreme dimensions is, of course, also measura- 
bly true of those of inferior elevation. The recent pho- 
to-spectrographic work of Hale and Deslandres suggests, 



SUN-SPOTS AND THE SOLAR SURFACE. 109 

however, a different explanation of the faculse. Their 
spectrum (as long ago frequently observed by the writer, 
visually) shows the great H and K bands of calcium 
always reversed by a thin bright line running down the 
middle of each ; and while the reversal directly over a 
spot is usually '' single," it is usually " double " ^ in the 
faculous region surrounding it — i. e., the bright line is 
itself double, as in Fig. 73, page 231. 

This makes it more or less probable that the faculse, 
instead of being mere protrusions from the photosphere, 
are really luminous masses of calcium vapor floating 
in the solar atmosphere — possibly, as Professor Hale 
thinks, identical with the prominences themselves. 

Fig. 81*. 




Spectboheliograph Photograph of Sfn's Disk, with Facul^. 

But Deslandres and Maunder dissent from this, and 
say that while these objects shown by the spectroscope 
are clearly connected with the prominences, they are 
as clearly not identical with them. 

* Such double reversal is a very common phenomenon in laboratory 
experiments upon metallic spectra. 



110 THE SUN. 

Fig. 31^ is from one of Professor Hale's spectro- 
heliograpiiic photographs, made by the apparatus de- 
scribed in Chap. VI, page 233. 

The facalae are found to some extent over the whole 
surface of the sun, though only sparingly in the polar re- 
gions, but they are especially abundant in the immediate 
neighborhood of spots, as Fig. 30 well shows. There are, 
however, numerous faculse without neighboring spots. 

Except near the spots, the faculse change form and 1 1 
place, for the most part, rather slowly, persisting some- 
times for several days without any very apparent alter- ,, 
ation. Still, close observation and micrometric measure- f | 
ment will always detect some movement or deforma- 
tion, even within an interval of only an hour or two ; 
and near the spots the changes are often so rapid and 
extreme as to puzzle even a skilled draughtsman to 
keep up with them. 

This, of course, shows that the faculse are not to be 
identified with mountains ; they are not permanent and 
stable, nor is the surface of the sun continental or oce- 
anic even, but either a sheet of flame or of cloud rolling 
and tossing, and never at rest. When we come to study 
the minute details of the granulation, we find move- 
ments at the rate of a thousand miles an hour to be the 
rule rather than the exception. 

And, although this is not the proper place to treat 
the subject at length, we may add that all we can learn 
as to the temperature and constitution of the sun makes 
it hardly less than certain that the visible surface, which 
is called the photosphere, is just a sheet of self-luminous 
cloud ; precisely like the clouds of our own atmosphere, 
with the exception that the droplets of water which 
constitute terrestrial clouds are replaced on the sun by 
drops of molten metal, and that the solar atmosphere in 




PHOTOGRAPHS OF A PORTION OF THE SUN, 

BY JANSSEN. 
Meudon, June 1, 1878. Intet-val. r,0 minnteR. 



SUN-SPOTS AND THE SOLAR SURFACE. m 

whicli they float is the flame of a burning, fiery furnace, 
raging with a fury and an intensity beyond all human 
conception. Looking at it ninety-three million miles 
away, we fail at first to see, in such objects as f acute and 
granules, the evidence of such commotion ; but, when 
we convert our micrometric measurements of barely 
perceptible changes into miles and velocities, and fig- 
ure to ourselves the scale of movement, we gradually 
comprehend their meaning, and begin to understand 
what we are dealing with. 

A great advance in our knowledge of the structure 
of the solar surface was gained through the photograph- 
ic work of Janssen, mentioned in a previous chapter.* 
Many of his pictures (in which the disk of the sun 
measures about eighteen inches in diameter) show the 
details of the surface nearly if not quite as well as any 
visual observations; and with the advantage that, while 
the observer with the eye could only command a small 
field of view, one can, on these photographic plates, 
command the whole at once, and catch the relations of 
different parts. On examining one of these magnifi- 
cent plates, one is at first struck with a sort of " smudg- 
iness " (to use the expression of Mr. Huggins in de- 
scribing them), which might give the impression that 
it was not properly cleaned before coating with the 
collodion. A closer examination, however, shows that 
the peculiarity is not in the plate but in the image ; 
there are patches of clear definition, half an inch or so 
in diameter upon a picture of the size mentioned, sep- 
arated by streaks and patches where everything is in- 
distinct and confused. 

One might naturally attribute this to the disturbance 
of the air in the telescope-tube, and to clouds of vapor 

* See page 52. 



112 THE SUN. 

rising from the damp collodion surface when struck by 
the flash of sunlight during its exposure ; but Janssen 
has found that pictures taken in immediate succession 
show the same ''smudges" on the same parts of the! 
sun, which, of couise, would not happen if they were! 
the result of accidental currents of air or vapor in the I 
telescope-tube. He infers, therefore, that the phenome- 1 
non is solar, and has given, it the name of the ReseauX 
Photospherique^ or " Photospheric Eeticulation," sincej 
the streaks and patches of indistinctness cover the sur- 
face like a net. 

The discovery of this feature in the structure of the 
solar surface is among the most interesting and im- 
portant results of astronomical photography. 

While pictures taken in immediate succession ex- 
hibit the same details of reticulation, those taken at 
intervals of an hour or two show great changes, es- 
pecially near spots and f acute. We present on the 
opposite page a pair of such photographs, borrowed 
from the " Annuaire " of the Bureau of Longitudes for 
1879. The original pictures were taken by Janssen, at 
Meudon, on June 1, 1878, with an interval of fifty min- 
utes between them. They show clearly the peculiar 
characteristics of the reseau photospherique^ as well as 
the nature and extent of the changes which occur in so 
short a time. Compare, especially, the granulation in 
the lower right-hand corner of each picture, and imme- 
diately around the upper spot, remembering all the 
while that the scale of the picture is about forty-six 
thousand miles to the inch, and that the little spot at 
the top of the figure is nearly seven thousand miles in 
diameter. 

The idea of M. Janssen is that the regions of indis- 
tinctness are those where we look down upon the sur- 



SUN-SPOTS AND THE SOLAR SURFACE. 113 

face through a portion of the sun's atmosphere which 
is at the moment especially agitated, while the parts 
where the details of the granulation are clear and well 
defined are those which, at the moment, are covered 
by an atmosphere unusually quiet and homogeneous. 
These regions are continually interchanging with each 
other, just as areas of storm and fine weather sweep over 
the surface of the earth, but with inconceivably greater 
swiftness. 

It is not, however, certain that the disturbed por- 
tions of the solar atmosphere, which produce the in- 
distinctness in question, lie near the sun's surface. It 
may be that they are high up, and it would not be an un- 
reasonable conjecture to suppose that the streamers and 
luminous masses of the corona may be concerned in the 
phenomenon ; it is almost certain that any great aggrega- 
tion of chromospheric matter would modify the appear- 
ance of whatever might be situated beneath it. The 
simple fact is, of course, that we are looking down upon 
the granules and other features of the sun's surface, not 
through an atmosphere shallow, cool, and quiet like the 
earth's, but through an envelope of matter, partly gase- 
ous and partly, perhaps, pulverulent or smoke-like, many 
thousand miles in depth, and always most profoundly 
and violently agitated. 

But, if there happens to be a well-formed group of 
spots upon the solar surface, they will be sure to claim 
the attention of one who, for the first time, looks at the 
sun through the telescope, quite to the exclusion of 
everything else. The umbra, with its central nuclei, 
and overlying bridges, veils, and clouds ; the penumbra, 
with its delicate structure of filaments and plumes ; the 
surrounding faculae and the agitated surface of the pho- 
tosphere in the whole neighborhood of the disturbance ; 



114 THE SUN. 

above all, the continual cliange and progress of phe- 
nomena — combine to make a fine sun-spot one of the 
most beautiful and intensely interesting of telescopic 
objects. 

Even before the days of telescopes there are numer- 
ous records of dark spots seen by the naked eye upon 
the disk of the sun, especially in the annals of the Chi- 
nese. In the year 807 a. d., a large spot was visible in 
Europe for some eight days, and was supposed by many 
to be the planet Mercury, as was the case with a spot 
observed by Kepler in 1609 ; indeed, in all cases where 
such appearances were noted, they were invariably as- 
cribed to bodies intervening between the earth and the 
sun. The idea of such imperfections upon the disk of 
a celestial body was utterly repugnant to the theologi- 
cal philosophy of the middle ages, and was admitted 
only slowly and grudgingly even after the demonstra- 
tion of the fact was complete. 

In 1610 and 1611 the discovery seems to have been 
made independently by Fabricius, Scheiner, and Gali- 
leo — Fabricius, according to our modern rules of scien-| 
tific priority, being entitled to the credit as the first to 
publish the fact in a work, " De Maculis in Sole Obser- 
vatis,'' which appeared at Wittenberg in June, 1611. 
The discovery was, of course, a necessary corollary to 
the invention of the telescope, which first came into use 
in Holland in 1608 or 1609. Fabricius's first observa- 
tion was made in December, 1610. Galileo, in a letterl 
responding to the account of Scheiner's discovery, andl 
published early in 1612, claims to have seen the sun- 
spots with his newly-constructed telescope as early as ^ 
October, 1610. Scheiner appears to have first seen sun-I 
spots at Ingolstadt in March, 1611 ; but his ecclesiasti- 
cal superior warned him against believing his own eyes 






SUX-SPOTS AND THE SOLAR SURFACE. 115 

in opposition to the authority of Aristotle, and it was 
not nntil November and December that he published 
an account of the matter in three letters to one Welser, 
a burgomaster of Augsburg, some months after the work 
of Fabricius had been printed. There is no reason 
whatever to doubt the word of Galileo, and his experi- 
ence in losing the credit of this discovery, in conse- 
quence of his slowness of publication, seems to have 
been the origin of his curious method of publishing his 
subsequent discoveries in the form of anagrams, the in- 
terpretation of which was withheld for a time. 

At the very outset of his observations, Fabricius, as 
well as Galileo, recognized that the spots are objects 
upon the surface of the sun, and that this body rotates 
on its axis, carrying them with it. Scheiner at first 
maintained that they were planets moving very near 
the sun, but not in contact with it. Many shared this 
opinion, and Tarde, a French astronomer, went so far 
as to name them the Bourbonian stars, in honor of the 
Bourbon dynasty. Scheiner's further observations soon 
convinced him, however, of the correctness of Galileo's 
opinion and arguments. Some twenty years later 
Scheiner published an enormous volume, the "Rosa 
ITrsina," containing an account of his observations and 
apparatus. His telescope was mounted equatorially, and 
arranged to throw the sun's image upon a screen in pre- 
cisely the manner employed by some of the best mod- 
ern observers. He determined the time of the sun's 
rotation and the position of his equator with a very 
creditable degree of accuracy. 

Since then observations upon these objects have 
been more or less kept up all the time, but not with 
any regular assiduity until within the last thirty years. 
It was soon found that they are only transitory and 



116 THE SUK 



lere-w 



cloud-like in their nature, and interest in them there-] 
fore flagged, until their relations to the constitution of 
the sun began to be recognized. 

A well-formed solar spot consists, generally speak- 
ing, of two portions — a very dark, irregular, central 
portion called the umbra, surrounded by a shade or 
fringe called the penumbra, less dark, and for the most 
part made up of filaments directed radially inward. 
The appearance of things, under ordinary circumstances 
of seeing, is as if the umbra were a hole, and the pe- 
numbral filaments overhung and partly shaded it from 
our view, like bushes at the mouth of a cavern. I say 
as if^ and very possibly this is the actual case, the cen- 

FiG. 32. 




Spot of July 16, 1866. 



tral portion being a real cavity filled with less luminous 
matter, and depressed below the general level of the 
photosphere, while the penumbra overhangs the edge. 

The figure, copied from Secchi, is a fair represen- 
tation of such a spot, and may be compared with the 



SUN-SPOTS AND THE SOLAR SURFACE. HY 

photographs of Janssen, which exhibit pretty much the 
same peculiarities, though with less of minute detail. 
The drawings of Nasmyth and Langley * show so much 
more of the detail than is ordinarily seen, that they are 
really less satisfactory representations of what one may 
expect when he observes a spot for the first time. Sev- 
eral points at once strike the attention. In the first 
place, the nearly circular form of the spot, which is the 
ordinary form daring the middle life of one of these 
objects. While forming, and when on the point of dis- 
appearing, it is usually much more irregular. It is to 
be noticed also that there is nothing like a gradual 
shading off, either between the umbra and the penum- 
bra or between the penumbra and the surrounding por- 
tions of the photosphere ; on the contrary, the line of 
separation is strongly marked in each case, the penum- 
bra being much brighter at the inner edge, and darker 
at the outer, so that it contrasts distinctly both with the 
umbra and with the neighboring surface of the sun. 
This brightness of the inner penumbra seems to be due 
to the crowding together of the penumbral filaments 
where they overhang the umbra. Again, it is observ- 
able that there is a general antithesis between the irreg- 
ularities of the contour of the outer and inner edges of 
the penumbra. For the most part, where an angle of 
the penumbral matter crowds in upon the umbra, it is 
matched by a corresponding outward extension into the 
photosphere, and viee vei'sa. It is noticeable also that 
many of the penumbral filaments are terminated by 
little detached grains of luminous matter, and there are 
also fainter veils of a substance less brilliant, but some- 
times rose-colored, which seem to float above the um- 
bra. Otherwise the umbra in the figure appears to be 

* See frontispiece, and page 104. 



118 THE SUN. 

uniformly dark ; ^ but, if we had been actually observing 
the object on the 16th of July, 1866, when this pict- 
ure was made, we should have found even the umbra 
full of detail — made up of cloudy masses of a brilliance 
really intense, and dark only by contrast with the still 
intenser brightness of the solar surface, as becomes ap- 
parent when the light from other portions is excluded. 
Probably we should have been able also to detect 
among these clouds one or more of the minute circular 
spots, first discovered by Dawes, much darker than the 
rest of the umbra, and looking like the mouths of tubu- 
lar orifices penetrating to unknown depths. 

If we were able to continue our watch for some 
time, we should see the details continually changing. 
The faint veils of overlying cirrus would probably melt 
away, and be replaced by others in some different po- 
sition ; the bright granules at the tips of the penumbral i 
filaments would seem to sink and dissolve, and fresh 
portions would break off to replace them. We should ' 
find a continual indraught of the luminous matter over 
the whole extent of the penumbra. Almost certainly 
the spot would change its form and size, quite percep- 
tibly from day to day, and sometimes even from hour 
to hour. Of course, we should find it steadily moving 
over the solar disk from the east toward the west, and 
as it neared the edge it would become apparently ellip- 

^ The umbra appears not black, but of a deep purplish tint. It is 
questionable, however, whether this color is real, or only due to the sec- 
ondary spectrum of the telescope object-glass. The principal reason for 
suspecting this to be the case is in the fact that, during a transit of 
Mercury or Venus, the planet's disk is found to present precisely the 
same tint, while there is no imaginable explanation for its really being 
anything but black. It is certain, too, on optical grounds, that any 
ordinary object-glass must show a purplish fringe extending inward over 
any dark spot upon a white background. 



SUN-SPOTS AND TEE SOLAR SURFACE. ng 

tical in form ; the penumbra on the edge of the spot 
nearest the center of the sun would grow narrower 
and, perhaps, disappear entirely, and at last the spot, 
appearing like a mere line of darkness, but probably 
accompanied by an attendant crowd of faculse, would 
pass out of sight behind the limb, perhaps to reappear 
again after a fortnight at the eastern edge. I say per- 
haps, because, quite as often as not, these short-lived 
objects are seen but once, not lasting through even a 
single revolution of the sun. 

The average life of a sun-spot may be taken as two 
or three months; the longest yet on record is that of 
a spot observed in 1840 and 1841, which lasted eigh- 
teen months. There are cases, however, where the dis- 
appearance of a spot is very soon followed by the ap- 
pearance of another at the same point, and sometimes 
this alternate disappearance and reappearance is several 
times repeated. While some spots are thus long-lived, 
others, however, endure only for a day or two, and 
sometimes only for a few hours. 

The spots usually appear not singly, but in groups — 
at least, isolated spots of any size are less common than 
groups. Very often a large spot is followed upon the 
eastern side by a train of smaller ones ; many of which, 
in such a case, are apt to be very imperfect in structure, 
sometimes showing no umbra at all, often having a pe- 
numbra only upon one side, and usually irregular in 
form. It is noticeable, also, that in such cases, when 
any considerable change of form or structure shows 
itself in the principal spot of a group, it seems to rush 
forward (westward) upon the solar surface, leaving its 
attendants trailing behind. When a large spot divides 
into two or more, as often happens, the parts usually 
seem to repel each other and fly asunder with great 



120 THE SUX. 

velocity — great, that is, if reckoned in miles per hour, 
though, of course, to a telescopic observer the motion 
is very slow, since one can only barely see upon the 
sun's surface a change of place amounting to two hun- 
dred miles, even with a very high magnifying power. 
Velocities of three or four hundred miles an hour are 
usual, and velocities of one thousand miles, and even 
more, are by no means exceptional. 

At times, though very rarely, a different phenome- 
non of the most surprising and startling character ap- 
pears in connection with these objects : patches of in- 
tense brightness suddenly break out, remaining visible 
for a few minutes, moving, while they last, with veloci- 
ties as great as one hundred miles a second. 

One of these events has become classical. It oc- 
curred on the forenoon (Greenwich time) of Septem- 
ber 1, 1859, and was independently witnessed by two 
well-known and reliable observers, Mr. Carrington and 
Mr. Hodgson, whose accounts of the matter may be 
found in the monthly notices of the Royal Astronomi- 
cal Society for November, 1859. Mr. Carrington at 
the time was making his usual daily observation upon 
the position, configuration, and size of the spots by 
means of an image of the solar disk upon a screen, 
being then engaged upon that eight years' series of 
observations which lies at the foundation of so much 
of our present solar science. Mr. Hodgson, at a dis- 
tance of many miles, was at the same time sketching 
details of sun-spot structure by means of a solar eye- 
piece and shade-glass. They simultaneously saw two 
luminous objects, shaped something like two new moons, 
each about eight thousand miles in length and two thou- 
sand wide, at a distance of some twelve thousand miles 
from each other. These burst suddenly into sight at 



SUN-SPOTS AND THE SOLAR SURFACE. 121 

the edge of a great sun-spot, with a dazzling brightness 
at least five or six times that of the neighboring por- 
tions of the photosphere, and moved eastward over the 
spot in parallel lines, growing smaller and fainter, until 
in about five minutes they disappeared, after traversing 
a course of nearly thirty-six thousand miles. Their pas- 
sage did not seem in any w^ay to change the configura- 
tion of the spot over which they passed. Mr. Carring- 
ton found his drawing, which was completed just before 
they appeared, still quite correct after they had vanished. 
Of course, it is possible to question the connection be- 
tween this phenomenon and the spot near which it ap- 
peared ; but, as somewhat similar appearances have been 
seen by other observers since then, and always in the 
neighborhood of spots, it is probable that there is some 
relation in the case. Opinions have differed widely as 
to the explanation. Some have maintained that the 
phenomenon was simply due to the fall of a couple of 
immense meteors into the sun's atmosphere, others that 
it was caused by some sudden and powerful eruption 
from beneath, such as the spectroscope often reveals to 
us nowadays ; an eruption, however, of most unusual 
brilliance and violence, for not one of the outbursts since 
then observed by the spectroscope has ever been visible 
without its aid. 

The event occurred in the midst of a remarkable 
magnetic storm : from August 28th to September 4th 
there were auroras every night all over the world, and 
the earth currents were often so strong as greatly 
to interfere with telegraphic communication. On the 
night of September 1st, however, as Mr. Ellis has lately 
shown from the original records, the magnetic disturb- 
ance was not specially intense, so that the occurrence 
observed by Carrington and Hodgson could not have 



122 THE SUK 

been the cause of the magnetic storm — more likely it 
was a consequence, if there was any connection. 

There is no regular process for the formation of a 
spot. Sometimes it is gradual, requiring days or even 
weeks for its full development, and sometimes a single 
day suffices. Generally, for some time before the ap- 
pearance of the spot, there is an evident disturbance of 
the solar surface, manifested especially by the presence 
of numerous and brilliant faculge,^ among which, 
''pores" or minute black dots are scattered. These 
enlarge, and between them appear grayish patches, ap- 
parently caused by a dark mass lying veiled below a 
thin layer of luminous filaments. The veil grows grad- 
ually thinner, and vanishes, giving us at last the com- 
pleted spot with its perfect penumbra. The " pores," 
some of them, coalesce with the principal spot, some 
disappear, and others constitute the attendant train. 
When the spot is once completely formed, it assumes 
usually an approximately circular form, and remains 
without striking change until its dissolution. As its 
end approaches, the surrounding photosphere seems to 
crowd in upon and cover and overwhelm the penumbra. 
Bridges of light, often many times brighter than the 
average of the solar surface, push across the umbra, the 
arrangement of the penumbra filaments becomes con- 
fused, and, as Secchi expresses it, the luminous matter 
of the photosphere seems to tumble pell-mell into the 
chasm, which disappears and leaves a disturbed surface 
marked with faculse, which in their turn subside after a 
time. As intimated before, however, the disturbance is 
not unf requently renewed at the same point after a few 
days, and a fresh spot appears just where the old one 
was overwhelmed. 

* This is Secchi's view. Lockycr maintains that the spots appear be- 
fore the facula3. 



SUN-SPOTS AXD THE SOLAR SURFACE. 123 

We transcribe from a paper by the late Dr. Peters, 
of Hamilton College, a very graphic account of the ap- 
pearance and decay of certain snn-spots, based upon his 
observations at Ts'aples in 1845-'46. It is printed in 
Volume IX of the " Proceedings of the American As- 
sociation for the Advancement of Science." He says : 

** The spots arise from insensible points, so that the exact 
moment of their origin can not be stated; but thej grow very 
rapidly in the beginning, and almost always in less than a day 
they arrive at their maximum of size. Then they are stationary, 
I would saj in the vigorous epoch of their life, with a well-defined 
penumbra of regular and rather simple shape. So they sustain 
themselves for ten, twenty, and some even for fifty days. Then 
the notches in the margin, which, with a high magnifying power, 
always appear somewhat serrate, grow deeper, to such a degree 
that the penumbra in some parts becomes interrupted by straight 
and narrow luminous tracks — already the period of decadence is 
approaching. This begins with the following highly interesting 
phenomenon: Two of the notches from opposite sides step for- 
ward into the area, over-roofing even a part of the nucleus ; and 
suddenly from their prominent points flashes go out, meeting 
each other on their way, hanging together for a moment, then 
breaking off and receding to their points of starting. Soon this 
electric play begins anew and continues for a few minutes, ending 
finally with the connection of the two notches, thus establishing a 
bridge, and dividing the spot in two parts. Only once I had the 
fortune to witness the occurrence between three advanced points. 
Here, from the point A a flash proceeded toward B, which sent 
forth a ray to meet the former when this had arrived very near. 
Soon this seemed saturated, and was suddenly repelled ; however, 
it did not retire, but bent with a sudden swing toward C; then 
again, in the same manner, as by repulsion and attraction, it re- 
turned to B ; and, after having thus oscillated for several times, A 
adhered at last permanently to B. The flashes proceeded with 
great speed, but not so that the eye might not follow them dis- 
tinctly. By an estimation of time and the known dimension of 
space traversed, at least an under limit of the velocity may be 
found ; thus, I compute this velocity to be not less than two hun- 



124 THE SUN. 

dred millions of metres (or about one hundred and twenty thou- 
sand miles) in a second (sic), 

'^ The process described is accomplished in the higher photo- 
sphere, and seems not to affect at all the lower or dark atmos- 
phere. With it a second, or rather a third, period in the spot's 
life has begun, that of dissolution, which lasts sometimes for ten 
or twenty days, during which time the components are again sub- 
divided, while the other parts of the luminous margin, too, are 
pressing, diminishing, and finally overcasting the whole, thus end- 
ing the ephemeral existence of the spot. 

*' Kather a good chance is required for observing the remark- 
able phenomenon which introduces the covering process, since it 
is achieved in a few minutes, and it demands, moreover, a per- 
fectly calm atmosphere, in order not to be confounded with a 
kind of scintillation which is perceived very often in the spots, 
especially with fatigued eyes. The observer ought to watch for 
it under otherwise favorable circumstances when a large and ten- 
or twenty-days'-old spot begins to show strong indentations on 
the margin." 

Dr. Peters, so far as we know, is the only observer 
who describes the remarkable phenomenon of flashes 
extending across an umbra with electrical velocity ; and 
for this reason, and because his instrument was not of 
the highest power — a three-and-a-half-inch refractor — 
perhaps his account must be received with a little re- 
serve until further confirmed. At the same time, there 
is nothing in the nature of the sun, or of a sun-spot, so 
far as at present known, to make the statement in itself 
impossible ; and certainly Dr. Peters holds deservedly 
a very high rank among astronomers for acuteness and 
accuracy of observation and description. 

It must not be understood that the life-history of a 
spot, just sketched, applies to all, or even with exact- 
ness to a majority, of them. Almost every one has its 
own idiosyncrasies, departing in some respect or other 
from the usual course of things. Spots of nnusual mag- 



SUN-SPOTS AND THE SOLAR SURFACE. 125 

nitude and activity often seem to have no quiet middle 
life ; there is no time in their history when tliey are not 
doing something or other surprising, and more or less 
unprecedented. 

We have spoken of the filaments which compose 
the penumbra as directed inward toward the center of 
the spot. This is the general rule, but the exceptions 
are numerous, and nothing can show better than Pro- 
fessor Langley's exquisite drawing how wide the di- 
vergence often is from this law. While at the left- 
hand and upper portions of the great spot (which, 
though " typical," is not a specimen of a quiescent spot) 
the filaments present the ordinary appearance, at the 
lower edge and upon the great overhanging branch 
they are arranged very differently. Very curious, and 
rare^ also, though we have ourselves seen a similar thing 
on two or three occasions, is the feathery brush w^hich 
reaches in below the " branch," so closely resembling a 
frost-crystal upon the window-pane in a winter's morn- 
ing. What may be the cause of such formations it is 
now quite impossible to say. Probably analogies drawn 
from our terrestrial clouds will go further toward an 
explanation than any others yet proposed. 

Usually the penumbral filaments are brightest at the 
inner end where they apparently project over the um- 
bra, and under ordinary circumstances of vision the end 
appears blunt and even club-shaped. 

With the great twenty-three-inch telescope at Prince- 
ton, and on a few occasions, when the seeing has been 
fine enough to permit the use of powers of from six hun- 
dred and upward, the writer has found that, in many 
cases at least, the apparently club-like, almost bulbous, 
ends of the penumbral filaments are really fine, sharp- 
pointed hooks, reminding one of the curling tips of 



126 THE SUK 

flames, or grass-blades bending over. Ordinarily they 
are seen as club-like, simply because of their brightness 
and the irradiation and diflraction effects of moderate- 
sized object-glasses. 

Not unf requently the penumbral filaments are curved 
and spirally arranged, showing a marked cyclonic action. 
In such cases the whole spot usually turns slowly around, 
sometimes completing an entire revolution in a few days. 
More frequently, however, the spiral motion persists 
but a short time, and occasionally, after continuing for 
a while in one direction, the motion is reversed. Very 
often, in spots of considerable extent, there are opposite 
spiral movements in different portions of the umbra ; 
indeed, this is rather the rule than the exception. Neigh- 
boring spots show no tendency to rotate in the same 
direction. Tlie number of spots in which a decided! 
cyclonic motion appears is relatively quite small, not 
exceeding, according to the observations of Carrington 
and Secclii, more than two or three per cent, of the 
whole. Of course, these facts are sufficient to show 
that this kind of motion, when it occurs, is not attribut- 
able to anything like that action of the terrestrial atmos- 
phere which determines the right- and left-handed 
rotation of our great storms in the southern and northern 
hemispheres. It is probably caused in sun-spots by 
merely accidental circumstances which convert the pe- 
numbral indraught into a whirl of no great rapidity or 
certain direction. It does not seem possible to find in 
this occasional cyclonic motion, as Faye attempts to do, 
the key and explanation of the whole series of sun-spot 
phenomena. 

The dimensions of sun-spots are sometimes enor- 
mous. Many groups have been observed covering 
areas of more than one hundred thousand miles square, 



sun-kSpots and the solar surface. 127 

and single spots have been known to measure forty or 
fifty thousand miles in diameter, the central umbra 
alone being twenty-five or thirty thousand miles across. 
A spot, however, measuring thirty thousand miles over 
all, would be considered large rather than small. 

An object of this size upon the sun's surface can 
easily be seen without a telescope when the brightness 
is reduced either by clouds, or nearness to the horizon, 
or by the use of a shade-glass. At the transit of Venus, 
in 1882, every one saw the planet readily without tele- 
scopic aid. Her apparent diameter was about 67^' at 
the time, which is equivalent to about thirty one thou- 
sand miles on the solar surface. Probably a very keen 
eye would detect a spot measuring not more than twenty- 
three or twenty-four thousand miles. 

Hardly a year passes, at times when spots are numer- 
ous, without furnishing several as large as this ; so that 
it is rather surprising than otherwise that we have not 
a greater number of sun-spot records in the pre tele- 
scopic centuries. The explanation probably lies in two 
things : the sun is too bright to be often or easily looked 
at, and when spots were seen they would be likely to 
be taken for optical illusions rather than realities. 

During the years 1871 and 1872 spots were visible 
to the naked eye for a considerable portion of the time. 
On several occasions pupils of the writer have noticed 
them of their own accord, without having had their at- 
tention previously directed to the matter. 

The largest spot yet recorded was observed in 1858. 
It had a breadth of more than one hundred and forty- 
three thousand miles, or nearly eighteen times the diam- 
eter of the earth, and covered about one thirty-sixth of 
the whole surface of the sun. Other very large ones 
appeared in 1892 and 1893. 



128 



THE SUN. 



Fig. 33, taken by the publisher's permission from 
Flammarion's Popular Astronomy, represents a very 



Fig. 33. 




One of the Largest Sun-Spots : Seven Times the Size of the Earth. 
Observed October 14, 18S3. Visible to the Naked Eye. 

large and interesting spot which appeared in October, 
1883. It is from the drawings of Tacchini. Spet- 
tros. Ital., Yol. XIII. 

NATURE OF THE SPOTS. 

It has been intimated that the spots are depressions 
below the general level of the solar surface. For more 
than a century this has been the accepted doctrine, and 
it is probably correct ; at the same time it can hardly be 



SUN-SPOTS AND THE SOLAR SURFACE. 129 

regarded as absolutely settled, since it has been called in 
question recently by high authorities, and is still de- 
bated. In December, 1894, Mr. Howlett, who has been 
for more than thirty years a persistent observer of the 
sun, presented to the Royal Astronomical Society all 
his sun-spot drawings, several thousand in number, and 
covering the w^hole period from 1859 to 1893. He took 
the opportunity to express very strongly his opinion 
that the facts are against the theory that the spots are 
"hollows" as usually supposed, and was supported in 
his view by a number of good observers, who made it 
clear that if the spots are really depressed at all, they 
must be very shallow compared with their diameter. 

The idea was first clearly brought out by Dr. Wil- 
son, of Glasgow, in 1769, and his demonstration was 
based upon the behavior of the penumbra of a spot 
w^hich he observed in November of that year. He 
found that, when the spot appeared at the eastern limb 
or edge of the sun, just mxoving into sight, the penum- 
bra was well marked on the side of the spot nearest to 
the edge of the disk, while on the other edge of the 
spot, that next the center, there was no penumbra vis- 
ible at all, and the umbra itself was almost hidden, as 
if behind a bank. When the spot had moved a day's 
journey farther inward toward the center of the disk, 
the whole of the umbra came into sight, and the pe- 
numbra on the inner edge of the spot began to be visible 
as a narrow line. After the spot was well advanced 
upon the disk, the penumbra was of the same width all 
around the spot ; but, when the spot approached the 
sun's western limb, the same phenomena were repeated 
as at the eastern — that is, the penumbra on the inner 
edge of the spot narrowed much faster than that on 
the outer, disappeared entirely, and finally seemed to 
10 



130 THE SUN. 

hide from sight much of the umbra, nearly a whole day 
before the spot passed from view around the limb. Of 
course, it is hardly necessary to point out what the fig- 
ure at once makes evident, that this is precisely the way 
things would go if the spot were a saucer-shaped de- 



Fio 




Diagram illustrating the Fact that Sun-Spots are Hollows in thb 
Photosphere. 

pression in the sun's surface, the bottom of the saucer 
corresponding to the umbra, and the sloping sides to 
the penumbra. 

The observation of a single spot would hardly 
settle the question, because we frequently have spots 
with a one-sided penumbra. In fact, when spots are 
either in the process of formation or of dissolution 
the penumbra is seldom of uniform width all around. 
De La Rue, Stewart, and Loewy made, therefore, some 
years ago, a careful discussion of something more than 
six hundred cases of S2Dots, with measurable penum- 
brse, and found that, in a little over seventy-five per 
cent, of all the cases, the penumbra was widest on the 



SUN-SPOTS AND THE SOLAR SURFACE. 131 

edge of the spot nearest the limb, as Wilson's theory 
requires; in a little more than twelve per cent, there 
was no noticeable difference ; and in the remaining 
twelve per cent, it was widest on the inner edge. 
Father Sidgreaves, on the other hand, in following up 
the discussion raised by Mr. Howdett, gets an opposing 
verdict from the Stonyhurst sun-spot drawings. Out of 
one hundred and eighty-seven sketches, which he se- 
lected as fair tests of the Wilsonian theory, only forty- 
seven favored it, and one hundred and forty were 
opposed. But we suppose he has included as '^ opposed '' 
all that did not distinctly indicate depression. 

Others, Secchi especially, have investigated the mat- 
ter by carefully measuring, from day to day, the position 
on the sun's disk of some selected point in the umbra of 
a spot. The work is not easy, and rather unsatisfactory, 
on account of the rapid changes, which make it difficult 
to identify the point of reference in successive observa- 
tions ; still, the result appears decisive, showing, as an 
ordinary rule, that w^hat may be called the " floor " of 
the umbra is depressed from two to six thousand miles, 
and sometimes more, below the general level of the 
photosphere. But the refraction of the solar atmos- 
phere makes the result uncertain. 

On a few occasions, when a spot of unusual size and 
depth passes over the limb of the sun, a distinct depres- 
sion is observed in the outline. Cassini describes such 
an instance in 1719. Herschel, De La Rue, Secchi, and 
others have given us several other observations of the 
same kind. Usually, however, the faculse, which sur- 
round the spot, mask this effect entirely, and often 
actually give us a number of little projecting hillocks 
in place of the expected depression. 



132 THE SUN. 

SPECTRUM OF SUN-SPOTS. 

The spectrum of a sun-spot also furnishes an argu- 
ment in the same direction, tending to show that the 
dark portion is a cavity filled with gases and vapors, 
which produce the obscuration, in part, at least, by ab- 
sorbing the light emitted from the floor of the depres- 
sion. It is not difficult to set the instrument in such a 
manner that the image of a sun-spot shall fall precisely 
upon the slit of the spectroscope. In this case the spec- 
trum will be seen to be traversed by a longitudinal dark 
stripe, which is the spectrum of the umbra of the spot : 
on each side is the spectrum of the penumbra, which is 
usually only a trifle fainter than that of the general sur- 
face of the sun. The width of the stripe, of course, de- 
pends upon the diameter of the spot. Along the whole 
length of the spot-spectrum the background is darkened, 
showing a general absorption ; and in the upper part of 
the spectrum, from F to H, this seems to be pretty 
much all that can be noticed. The middle portion of 
the spectrum, however, under extremely high dispersion 
is dift'erent in this respect, as was discovered by the 
writer in 1883, and has since been abundantly confirmed 
by Duner and others. In many spots, especially large 
ones that are nearly circular and quiescent with a very 
dark nucleus, the spectrum of the nucleus between E 
and F is not continuous, but is made up of countless 
fine, dark lines, for the most part touching or slightly 
overlapping, leaving here and there, however, unoccu- 
pied intervals which look like (and may be) bright lines. 
Each dark line is spindle-shaped — i. e., thicker in the 
middle where the spectrum is darkest, and tapers to a 
fine, faint, hair-like mark at each end ; most of them 
can be traced across the penumbra-spectrum, and even 
out upon the general surface of the sun. The aver- 



SUX-SPOTS AND THE SOLAR SURFACE. I33 

age distance between the lines is about half that be- 
tween the two components of J,, so that within the h 
group the total number of dark lines is some 300, and 
there are seven or eight of the bright lines. This 
structure is most easily seen in the part of the spectrum 
between E and F ; above F the lines are crowded so 
closely that it is difficult to resolve them, and below E 
they appear to grow wider, more diffuse, and fainter. It 
seems to indicate that the principal absorption which 
darkens the center of a sun-spot is not such as would be 
caused by minute solid or liquid particles — by smoke or 
cloud — which would give a continuous spectrum ; but it 
is a true gaseous absorption, producing a veritable dark- 
line spectrum, in which the lines are countless and con- 
tiguous. 

In the lower part of the spectrum, especially between 
C and D, the spot-spectrum is .full of interesting details 
and peculiarities, which deserve a far more thorough 
and prolonged study than they have yet received. 
Many of the dark lines of the ordinary spectrum are 
wholly unmodified in the spectrum of the spot ; in fact, 
this seems to be the case with the majority of them. 
Others, however, are much widened and darkened, and 
some, which are hardly visible at all in the ordinary 
spectrum, are so strong and black as to be very conspicu- 
ous : these are usually spindle-shaped, much wider in 
the center of the nucleus than at its edges and in the 
penumbra, so that they are often called " fish-bellies." 
Certain other lines, which are strong in the ordinary 
spectrum, thin out and almost disappear in the spot- 
spectrum, and some are even reversed at times. There 
are also a number of hright lines, not very brilliant, to 
be sure, but still unmistaka?jle, and there are some dark 
shadings of peculiar appearance. 



lU 



TUE SUN. 



The annexed figure (Fig. 35), wliicli represents a 
small portion of the spectrum of a spot observed by the 
writer in 18Y2, shows nearly all of these peculiarities. 
The portion represented lies between C and D, the scale 
attached being that of Kirchhoff s map. 




Portion of Sun-Spot Spectrum between C and D. 

Speaking in a general way, the lines of hydrogen, 
iron, titanium, calcium, sodium, and vanadium are spe- 
cially affected. The hydrogen lines are often reversed ; 
those of iron, titanium, calcium, and vanadium are usu- 
ally thickened, and those of sodium are often enormously 
widened, and occasionally both widened and doubly 
reversed, as shown in Fig. 36, which represents their 
appearance in the spectrum of a spot observed on Sep- 
tember 22, 1870. It will be noticed that at the same 
time the helium-line, Dg, which usually is invisible on 
the solar surface, was quite conspicuous as a dark shade. 
On this occasion the lines of magnesium also behaved 
in the same manner as those of sodium. 

As has already been mentioned (page 109), the H 
and K bands are always reversed in the sun-spot spec- 
trum. Usually, over the spot itself, the reversal is 
only " single," but double reversal is not very uncommon. 



SUN-SPOTS AND THE SOLAR SURFACE. 



135 



Mr. Lockyer announces, as a result of a long series of 
observations, that there is a striking difference between 
the spot spectra at the time of maximum and minimum 
sun-spot frequency ; the lines that are most conspicuous 
by widening and darkening being by no means the same 
in the two cases. The most remarkable change is in the 
lines of iron, which are usually conspicuous, but almost 
vanish from the spot-spectrum at the sun-spot maximum. 
At times, also, the spectrum of a spot gives evidence 
of violent motion in the overlying gases by distortion 
and displacement of the lines. When the phenomenon 




Keveesal of the D-Lines. 



occurs, it is more usually at points near the outer edge 
of the penumbra than over the central portion of the 
spot ; but, o'ccasionally, the whole neighborhood is vio- 
lently agitated. In such cases it often happens that 
lines in the spectrum side by side are affected in en- 
tirely different ways — one will be greatly displaced, 
while its neighbor is not disturbed in the least, showing 
that the vapors which produce the lines are at different 
levels in the solar atmosphere, and do not participate 
to any great extent in each other's movements. 

It is an important fact that the same thing is often 
true of lines which are ascribed to a single substance : 
of two iron lines, for instance, one may be disturbed 



136 THE SIX. 

and another unaflfected. Mr. Lockyer lays great stress 
on this as supporting his dissociation hypothesis ; but 
other explanations are also available, see page 91. 

In a few instances the gaseous eruptions in the 
neighborhood of a spot are so powerful and brilliant 
that, with the spectroscope, their forms can be made 
out on the background of the solar surface in the same 
way that the prominences are seen at the edge of the 
sun. In fact, there is probably no difference at all in 
the phenomena, except that only prominences of most 
unusual brightness can thus be detected on the solar 
surface. An occurrence of this kind fell under the 
writer's observation on September 28, 1870. A large 
spot showed in the spectrum of its umbra all the lines 
of hydrogen, magnesium, sodium, and some others, re- 
versed. Suddenly the hydrogen lines grew greatly 
brighter, so that, on opening the slit of the spectroscope, 
two immense luminous clouds could be made out, one of 
them nearly 130,000 miles in length, by some 20,000 in 
width, the other about half as long. They seemed to 
issue at one extremity from two points near the edge 
of the penumbra of the spot. After remaining visible 
about twenty minutes, they faded gradually away, with- 
out apparent motion. 

In addition to spots, such as we have been dealing 
with, there are occasionally seen on the solar surface 
dark-gray patches, which Trouvelot, who first called 
attention to them in 1875, has named " veiled spots," ^ 
considering that they are essentially of the same nature 
as other spots, but differing in this, that the disturb- 
ance which generates them is not sufficiently powerful 
to reach the surface and break entirely through the 

* For Trouvelofs account of them, see " American Journal of Science 
and Art," March, 1876, Third Series, vol. xi. 



SUX-SPOTS AND THE SOLAR SURFACE. 137 

photosphere. Over these veiled spots the bright gran- 
ules are less numerous and smaller than elsewhere, but 
much more mobile ; sometimes, and frequently indeed, 
they are overlaid by faculse. The changes of form and 
appearance in these objects are very rapid, affairs of a 
minute or two only, according to Trouvelot. They are 
found all over the solar surface, not being at all con- 
fined to the regions occupied by the ordinary spots, 
but sometimes occurring within eight or ten degrees of 
the sun's pole. They have been little observed, how- 
ever, and information respecting them is as yet very 
meager. 

ROTATION OF SUN AND PROPER MOTIONS OF SPOTS. 

We have already mentioned that the spots travel 
across the disk of the sun, from the eastern edge to 
the western, in such a manner as to show that they are 
attached to the surface, and that the sun rotates upon 
its axis. The true period is about twenty-five days,"^ 
the apparent or " synodic " period being some two days 
longer, because the earth itself is continually moving 
forward in its orbit. 

AVhen we come, however, to study the motions of 
the spots more carefully, we find that they have move- 
ments of their own {proper motions^ as astronomers call 
them), both in latitude and longitude, so that no observa- 
tions of any single spot, however carefully conducted, 

* It is perhaps worth noting that, between the sun and the earth's 
magnetism, there is an unquestionable, though still unexplained, connec- 
tion, which shows itself in many ways. Among the numerous periodic 
variations of this magnetism Uornstein finds one with a period of 26*32 
days. Ass^iming this to be due to the sun's synodic rotation, he gets 
24*55 days for the true rotation. Very similarly Bigelow deduces 24*86. 
Veeder's "aurora period" (27*28 days) gives 25*38 — all of which may 
be taken for what it is worth. 



138 THE SUN. 

can furnish an accurate determination of the position of 
the sun's axis and its period of rotation. This fact does 
not seem to have been comprehended by the early ob- 
servers (though a neglected remark of Scheiner's indi 
cates that he had a glimpse of the truth), and hence we 
have serious discordances between their different results, 
which range from 25*01 days, the result obtained by 
Delambre in 1775, to 25*58 days, as determined by 
Cassini about a hundred years earlier. The different! 
values for the inclination of the sun's equator to the 
ecliptic lie between 6|^° and 7^°, and those for the lon- 
gitude of the node between 70° and 80°. The most 
reliable recent results are those of Carrington and 
Spoerer. The former makes the "inean period of the 
sun's rotation 25*38 days, while Spoerer gives it as 
25*23. 

THE EQUATORIAL ACCELERATION. 

The researches of Carrington,^ between 1853 and 
1861, first brought out clearly the fact that, strictly 
speaking, the sun, as a whole, has no single period of 
rotation, but different portions of its surface perform 
their revolutions in different times. The equatorial re- 
gions not only move more rapidly in miles per hour 
than the rest of the solar surface, but they cor)iplete the 
entire rotation in shorter time. If we deduce the 
period by means of spots near the sun's equator, we 
shall find it to be very nearly 25 days, a trifle less — 
according to Carrington. Spots at a solar latitude of 

*A memoir by Laugicr, presented to the French Academy in 1844, 
but never published m extenso^ contains, according to Faye, data which 
would lead to the same result. The summary, given in the " Comtes 
Rendus," fails, however, to indicate any appreciation of the systematic 
variation of rotation rate from equator to poles, and in no way invali- 
dates Carrinpjton's claim to be considered the discoverer of the law. 



SUN-SPOTS AND THE SOLAR SURFACE. 139 

20^ have, on the other hand, a period nearly 18 hours 
longer; at 30° the period rises to 26^ days, and at 45° 
to 27|, though in this latitude there are so few spots 
that the determination is not very reliable. Beyond 
this latitude we have nothing satisfactory, and it is not 
possible to determine, with any certainty, whether this 
retardation continues to the pole or not. 

It is a curious circumstance, probably connected 
with this remarkable law of surface-movement, that the 
spots mostly lie between ten and thirty-five degrees 
of latitude on each side of the sun's equator ; and it is 
this fact which makes it difficult to ascertain the exact 
laws of the solar rotation, since our observations are 
confined to such a limited range of latitude. As yet, 
no points have been found near the sun's poles perma- 
nent and definite enough to permit precise observations 
I covering a sufficient interval of time. 

By a discussion of all his observations, more than 
5,000 in number, of 954 different groups of spots, Mr. 
Carrington deduced the expression X = 865'— -165^ sin^Z 
for the daily motion of the surface of the sun in dif- 
ferent solar latitudes, I representing the latitude in the 
formula, and X the daily motion in minutes of solar 
longitude. This, as was said before, would make the 
rotation period of the sun's equator a little less than 
25 days. The expression, however, is purely empirical, 
and no imaginable theoretical explanation can be given 
for the fractional exponent ^, 

Faye, assuming on theoretical grounds that this ex- 
ponent ought to be 2, finds from the same observations 
the formula X = 862'— 186' sin^Z, an expression which 
agrees with all but a few of the observations nearly as 
well as Carrington's. 

Spoerer, from observations of his own, made be- 



140 THE SUN. 



t 



tween 1862 and 1868, and combined with those of 
Secchi and others, derives the still different formula, 
X = 1011' - 203' sin(41° 13'+ T). 

Tisserand, from observations of 325 spots in 18Y4- 
'75, deduces the expression X = 857'-6 — 157'-3 sin7. 
But this is probably less reliable than either of the pre- 
ceding, being founded on a much smaller number of ob- 
servations. 

Wilsing, of Potsdam, in 1888 published a discussion 
of several hundred faculce shown on their phothelio- 
graph plates, and deduced a rotation-period of 25*23 
days ; but he found no indications of equatorial acceler- 
ation, and concluded that this peculiarity of the photo- 
sphere, where the spots have their residence, does not 
extend to the region of the faculae — a very perplexing 
fact, if real. Still more recently, however, Stratonoff, of 
Pulkowa, from a discussion of their plates, finds from 
the faculse a result quite in accordance with those of 
Carrington and Spoerer. 

We have already referred to the evidence of the 
sun's rotation given by the spectroscope (page 100), 
and have specially quoted the remarkable work of 
Duner. His results show (we think conclusively, 
though objections have been raised in certain quar- 
ters) that tlie region in which the dark lines of the 
spectrum originate share perfectly the motion of the 
photosphere. His observations, moreover, have this 
great advantage over those made on spots and faculae, 
that they extend as far as 75° on each side of the sun's 
equator. The observations are very well represented by 
the equation X = 846'— 272'-4 sin7. This would cor- 
respond to a rotation-period of 25*53 days at the sun's 
equator and about 37*5 at the pole — but the polar 
period is very uncertain. 



SUN-SPOTS AND TEE SOLAR SURFACE. 141 

While either of the formulae given above agrees 
fairly with the facts observed, neitlier of them can be 
regarded as logically established upon a sound physical 
explanation. 

The cause of this peculiar surface-drift is not yet 
known. Sir John Herschel was disposed to attribute 
it to the impact of meteoric matter striking the sun's 
surface mainly in the neighborhood of the equator, and 
so continually accelerating its rotation, as a boy's peg- 
top is whipped up by the skillfully applied lash. Per- 
haps there is nothing absurd in the idea that a sufficient 
quantity of meteoric matter may reach the sun, or that 
the meteors move, for the most part, in the plane of 
the sun's equator, and direct, i. e., %oiih and not against 
the motion of the planets — so that their fall would be 
mostly confined to the equatorial regions, and would 
thus hasten, and not retard, the surface motion. 

But then the duration of- the sun's rotation period 
should continually grow shorter, an effect which does 
not appear from a comparison of Scheiner's results with 
those most recently obtained. Of course, it may be 
that such an acceleration has actually occurred, only too 
small to be yet detected ; still, it would seem probable 
that any "driving," sufficient to establish nearly two 
days' difference between the rotation periods at the 
equator and at latitude 40°, must have produced a very 
sensible effect within three hundred years. 

It is more probable that the equatorial acceleration 
is connected in some way with the exchange of matter 
which, if the sun is for the most part gaseous, as now 
seems likely, must continually be going on between the 
outside and inside of the globe. If the photosphere is 
formed of masses falling^ such an effect would be a 
necessary consequence. If we suppose that the out- 



142 THE SUN. 

rushing streams of heated gas and vapor, as they rise, 
continue in the gaseous condition until they reach the 
summit of their ascent, and remain at this height long 
enough to acquire sensibly the rotation velocity corre- 
sponding to their altitude, and that then the products of 
condensation, resulting from their cooling, fall down- 
ward, and thus falling constitute tlie photosphere, we 
should have precisely the actual phenomenon. The ro- 
tation velocity of each visible element of the photosphere 
would be that corresponding to a greater altitude, and 
therefore greater than that naturally belonging to its 
observed position, and this difference would vary from 
the equator, where it w^ould be a maximum, to the poles, 
where it would vanish. 

Of course, it is not necessary to such an effect that 
the conditions supposed should be rigidly complied 
with ; it will suffice to admit that in the photosphere 
the falling masses are more conspicuous than those 
which are ascending or stationary, and it would seem 
hardly possible that it should be otherwise. Whether, 
however, the effect thus produced would account in 
measure as well as kind for the observed phenomena, 
is a question requiring for its answer a more thorough 
mathematical investigation than the waiter has yet been 
able to undertake. 

If we consider only the spots^ it would seem entirely 
possible that they may be produced by matter which 
has fallen from a height of even fifteen or twenty thou- 
sand miles, and that fall would be quite sufiicient to ac- 
count for their whole acceleration. 

The fact that rapid changes in the configuration of 
a spot are generally accompanied by an eastward rush 
of the whole, also favors the idea that a downfall of 
something from above is concerned in the matter. 



SUN-SPOTS AND THE SOLAR SURFACE. I43 

If we rightly understand the matter, this theory of 
the equatorial acceleration is in substantial accordance, 
so far as it goes, with that formulated some years later 
by Mr. Lockyer, and given in the last chapter of his 
" Chemistry of the Sun." Bat his " dissociation theory " 
apparently has an important role to play in providing 
the " hundreds of millions of tons " of falling matter 
that produce the phenomena by their " down rush." 
Schaeberle also attributes the equatorial acceleration to 
the falling back of material that has been projected to 
a great elevation above the photosphere. 

The idea of Faye appears to have been nearly the 
reverse of that here suggested. He attributes the for- 
mation of the photosphere to gaseous matter not falling 
from above, but ascending from lelow^ and starting 
from a stratum at a certain depth below the surface ; 
by supposing the depth of this stratum to vary with 
the latitudes, being greatest at the poles of the sun and 
least at the equator, it is easy to explain on this hy- 
pothesis the accelerated motion of the surface at the 
equator, and to justify his formula, which makes the 
retardation at higher latitudes proportional to the square 
of the sine of the latitude ; but no reason is evident why 
the depth of this stratum should vary. 

Certain later investigations in 1886 upon the rota- 
tion of fluid masses, by Jukowsky, of Moscow, as ap- 
plied to solar conditions by his colleague Belopolsky, 
perhaps warrant a hope that the phenomena of surface- 
drift in longitude, and even the periodicity of the spots, 
ultimately find a rational explanation as necessary re- 
sults of the slow contraction of a non-homogeneous and 
mainly gaseous globe. The subject is difficult and 
obscure ; but if it can be proved, as seems not impossi- 
ble, that, on mechanical principles, the time of rotation 



14J: THE SUN. 






of the central portions of such a whirling mass must 
be shorter than that of the exterior, tlien there will be, 
of necessity, an interchange of matter between the 
inside and outside of the sphere, a slow surface- 
drift from equator toward the poles, a more rapid intef^\ 
not current along and near the axis, from the poles 
toward the equator, a continual '^ boiling up " of internal 
matter on each side of the equator, and, finally, just 
such an eastward drift near the equator as is actually 
observed. Moreover, the form of the mass, and the 
intensity of the drift and consequent " boiling up " from 
underneath might, and probably would, be subject to 
great periodical variations. 

As to Zollner's idea that the equatorial acceleration 
is due to the friction between a liquid sheet, constitut- 
ing the photosphere, and a solid nucleus below, it is 
hardly necessary to say that this view is in complete 
opposition to those held by almost all astronomers, and 
seems to be untenable in its fundamental assumptions. 

On the whole, however, the writer sympathizes with 
Duner in his conclusion : '^ I must confess that this dif- 
ference between the rotation periods in* the different 
(solar) latitudes appears to me incomprehensible, and 
constitutes one of the most difficult problems of astro- 
physics." No theory yet presented is really satisfactory. 

THE POSITION OF THE SUN^S AXIS. 

The plane of the snn's rotation is slightly inclined 
to that of the earth's orbit. According to Oarrington, 
the angle is 7° 15', while Spoerer makes it 6° 57'. This 
plane cuts the ecliptic at two opposite points called the 
nodes, one of which is in longitude 73° 40', according to 
Oarrington, and 74° 36' according to Spoerer. The 
axis of the sun is therefore directed to a point in the 



SUN-SPOTS AND THE SOLAR SURFACE, 



145 



constellation of Draco, not marked by any conspicuous 
star. Astronomers define its position by saying that 
its right ascension is 18^ 44"", and its declination is 64°. 
It is almost exactly half-way between the bright star 
a Lyrse and the polar star. 

The earth passes through the two nodes on or about 
the 3d of June and the 5th of December. At these 
times the spots move apparently in straight lines across 
the sun's disk, and his poles are situated on its circum- 
ference. During the summer and autumn, from June 
to December, the sun's northern pole is inclined toward 
the earth ; during the winter months, the southern. The 
angle which the sun's axis appears to make with a north 
and south line in the sky (technically, the position-angle 
of the sun's axis) changes considerably during the year, 
varying 26° each side of zero. As it is often very 
desirable for an amateur to know this angle approxi- 
mately, we insert the following little table, giving the 
position angle of the sun's north pole referred to the 
center of the disk. The table is derived from the much 
more extensive one in Secchi's " Le Soleil " : 

POSITION ANGLE OF SUN»S AXIS. 
January 4, July 6 0°00. 



Jan. 15^ June 25 


6° west. 


Dec. 24, July 17... 


5° east. 


Jan. 26, June 14 


10° west. 


i Dec. 15, July 29... 


10° east. 


Feb. Y, June 2 


15° west. 


Dec. 3, Aug. 11 


15° east. 


Feb. 22, May 18 


20° west. 


Nov. 19, Aug. 27... 


20° east. 


March 18, April 26.. 


25° west. 


Oct. 29, Sept. 20... 


25° east. 


April 5 


26° 20' west. 


Oct. 10 


26° 20' east. 







It is understood, of course, that the table is only 
approximate, because the numbers change slightly ac- 
cording to the place of the current year in the leap- 
year cycle ; but the results obtained from it are always 
11 



146 



THE SUN. 



correct within about J^, which is near enough for most 
purposes. 

Fig. 37 illustrates these points, giving the position- 
angle of the sun's axis, and the aspect of his equator at 
different times of the year as seen from the earth. For 



Fig. 37. 





APRILS, N. 26k W. 





OCT. 10, N. 26'/? E. 






SEPT. 4. DEC. 5. MARCH 7. 

Position- Angle of Sun^s Axis, and Aspect of ms Equator. 

the sake of clearness, however, the inclination of the 
sun's equator to the ecliptic is considerably exaggerated 
in the lower row of figures : the equator never appears 
so strongly curved as there represented. 



PROPER MOTION OF SPOTS. 

After making due allowance for the equatorial 
acceleration, it is found that almost every spot has more 
or less motion of its own. Between latitudes 20° north 
and 20° south, Mr. Carrington finds, on the whole, a 
slight tendency to motion toward the equator, the move- 
ment amounting to a minute or two of arc per diem ; 
from 20° to 30° on both sides of the equator, there is 



SUN-SPOTS AND THE SOLAR SURFACE. I47 

a somewhat more decided motion toward the poles. 
Faye has also shown that many spots move in small 
ellipses upon the surface of the sun, completing their 
circuits in a day or two, and repeating them with great 
regularity for weeks, and even months. Whenever a 
spot is passing through sudden changes, it generally 
moves forward upon the solar surface, as has already 
been mentioned, with something like a leap ; and, when 
a spot divides into two or more, the parts generally sep- 
arate with a very considerable velocity, as if (we do not 
say hecause) there was a repulsion between them. 

DISTRIBUTION OF SUN-SPOTS. 

The sun-spots, as has already been said, are not dis- 
tributed over the sun's surface with anything like uni- 
formity. They occur mainly in two zones on each side 
of the equator and between the latitudes of 10° and 30°. 
On the equator itself they are comparatively rare ; there 
are still fewer beyond 35° of latitude, and only a single 
spot has ever been recorded more than 45° from the 
solar equator — one observed in 1846 by the late Dr. 
Peters, then in Naples. 

The figure shows the distribution of 1,386 spots ob- 
served by Carrington. The figure is constructed in 
this way : The circumference of tlie sun, on the left- 
hand side of the figure, is divided into five-degree 
spaces from the equator each way, and at each of them 
is erected a radial line whose length mfour hundredths 
of an inch is proportional to the number of spots ob- 
served within 2|^° of latitude on each side. Thus, the 
line drawn at 20° north latitude, and marked " 151," is 
\^ of an inch long, and means that 151 spots were 
recorded between 17^° and 22^° north latitude. 

It is at once evident from mere inspection that the 



148 



THE SUN. 



distribution follows no simple law of latitude. On the 
northern hemisphere, the distribution, during the eight 
years over which the observations extend, was not very 
irregular, though there is a distinct minimum at 15°, 



Fig. 88. 




DlSTRlBtTTlON OF SUM-SPOTS AND PROTUBERANCES. 

and two maxima at about 11*^ and 22° of latitude. On 
the southern hemisphere the minimum at 15° is very 
marked, and the numbers at 10° and 20° are far in 
excess of those in the northern hemisphere. Of the 
whole number, 711 were in the southern hemisphere, 
as against 675 in the northern. 

The minimum at 15° of latitude was special to the 
date of observation, and had its origin in a law discov- 
ered by Spoerer a few years ago — to be discussed later 
(page 156). His own observations from 1861 to 1867 
show nothing of the kind. They give the following 



SUN-SPOTS AND TDE SOLAR SURFACE. I49 

distribution of 1,053 spots in latitude, viz. : -[- ^5°, 4 ; 
+ 30^ 4 ; + 25°, 16 ; + 20^ 50 ; + 15°, 133 ; + 10°, 
198 ; -\- 5°, 114 — in all, 519 spots north of the solar 
equator. 40 sjDots were on the equator, or within 2° of 
it. South of the equator w^e have, in latitude : — 5°, 
113 ; - 10°, 206 ; - 15°, 109 ; - 20°, 38 ; - 25°, 19 ; 
- 30°, 7 ; - 35°, 1 ; - 40°, 1— in all, 494 southern 
spots. In 1866, a year of spot minimum, there were 
only 94 spots in all, and of these 94 all but two were 
situated within 17° of the equator. 

It is to be noticed that at times when spots are 
abundant their mean latitude is greater than w^hen they 
are few, or, in other words, an increase in the number 
of spots generally carries with it a widening of the zones 
in which the spots appear. All the observations concur 
in showing this. 

The cause of this distribution of the spots in zones 
is not known. It is probably connected with the origin 
of the spots themselves, and very possibly has something 
to do with the law of surface-motion just discussed. At 
least it is certain, as Faye pointed out some years ago, 
that, while at the solar poles and equator adjoining por- 
tions of the photosphere have no relative motion with 
reference to each other, yet in the middle latitudes 
this is not true ; here each element of the surface has a 
different velocity from those immediately north and 
south of it, so that they drift by each other like the 
filaments of a liquid current which is suffering retarda- 
tion, producing, as Faye supposes, whirlpools and eddies 
which, according to his view, generate the spots. 

As regards the sun's northern and southern hemi- 
spheres, there is often a great inequality. Thus, from 
1672 to 1704 absolutely no spots were recorded in the 
northern hemisphere, and when a few appeared in 1705 



]50 THE SUN. 

and 1714, the French Academy was formally notified of 
the fact as something very remarkable. We do not 
know that anything quite like this has ever happened 
since ; but the inequality between the two hemispheres 
is often very marked for months together, though in 
the long run there seems to be no difference. 

It is a question of much theoretical importance 
whether spots do or do not appear repeatedly at the 
same points; for if this is really the case, it would 
make it almost certain that below the photosphere there 
must be a coherent nucleus, carrying with it in its rota- 
tion such volcanic or otherwise peculiar regions as to 
cause the breaking out of spots above them. There 
would be no difficulty in accounting for two or three 
dissolutions and reappearances in the same region with- 
out any such hypothesis, since a great disturbance in 
the solar atmosphere would not subside entirely for a 
long time. The observations of Spoerer show that 
this actually happens, and that, for a period of several 
months, spots and faculse often recur several times at 
the same point. But his observations do not give any 
real support to the idea of a solid nucleus, nor has he 
himself ever favored such a view, although some (and 
among others the writer), misunderstanding certain ex- 
pressions of his, have supposed that he did. 



CHAPTER Y. 

PERIODICITY OF SUN-SPOTS; TUEIR EFFECTS UPON THE EARTH, 
AND THEORIES AS TO THEIR CAUSE AND NATURE. 

Observations of Schwabe. — Wolf's Numbers. — ^Proposed Explanations of 
Periodicity. — Connection between Sun-Spots and Terrestrial Magnet- 
ism. — Remarkable Solar Disturbances and Magnetic Storms. — Effect 
of Sun-Spots on Temperature. — Sun-Spots, Cyclones, and Rainfall. — 
Researches of Symons and Meldrum. — Sun-Spots and Commercial 
Crises. — Galileo's Theory of Spots. — Herschel's Theory. — Secchi's 
First Theory. — Zollner's. — Faye's. — Secchi's Later Opinions. — Theo- 
ries of Lockyer, Schaeberle, and others. 

It was early noticed that the number of sun-spots is 
very variable, but the discovery of a regular periodicity 
in their number dates from 1851, when Schwabe, of 
Dessau, first published the result of twenty-five years 
of observation. During this time he had examined the 
sun on every clear day, and had secured an almost per- 
fect record of every spot that appeared upon the solar 
surface. He began his work without any idea of ob- 
taining the result he arrived at, and says of himself, 
that, " like Saul, he went to seek his father's asses, and 
found a kingdom." His observations showed unmis- 
takably that there is a pretty regular increase and de- 
crease in the number of sun-spots, the interval from one 
maximum to the next being not far from ten years. 
Subsequent observations and a thorough examination 
of all known former records fully confirm this conclu- 



152 THE SUN. 



■ 

>me-S 



sion, except that the mean period appears to be some- 
what greater, eleven and one ninth years being the vahie 
at present generally received. Professor R. Wolf, of 
Zurich, has been especially indefatigable in his investi- 
gations upon this subject, and has succeeded in disinter- 
ring from all sorts of hiding-places a nearly complete 
history of tlie solar surface for the past hundred and 
fifty years. Among other things he finds among the 
unpublished manuscripts of Horrebow (a Danish as- 
tronomer who flourished a century ago) a distinct in- 
timation (in 1776) that zealous and continued observa- 
tion of the sun-spots might lead to " the discovery of a 
period, as in the motions of the other heavenly bodies," 
with the added remark that " then, and not till then, 
it will be time to inquire in what manner the bodies 
which are ruled and illuminated by the sun are influ- 
enced by the sun-spots " — alluding, perhaps, to certain 
ideas then, as now, more or less current, and illustrated 
by the attempt of Sir W. Herschel, a few years later, to 
establish a relation between the price of wheat and the 
number of sun-spots. 

Wolf has brought together an enormous number of 
observations, and with immense labor has combined 
them into a consistent whole, deducing a series of " rel- 
ative numbers," as he calls them, which represent the 
state of the sun as to spottedness for eyerj year since 
1745. His "relative number" is formed in rather an 
arbitrary manner from the observation of the spots : 
representing this number by /*, the formula is, r = 
k(f-\-10ff\ in which (/ is the number of groups and 
isolated spots observed, and /* the total number of 
spots which can be counted in these groups and singly, 
while A: is a coefficient which depends upon the ob- 
server and his telescope. Wolf takes it as unity for 



PERfODICITY OF SUN-SPOTS. 



153 




te 

ml 
el 



154 TEE SUN. 

himself, observing with a three-inch telescope and powe: 
of 64. For an observer with a larger instrument, k would] 
be a smaller quantity, while a less powerful instrumentj 
and less assiduous observer would receive a "^" greate: 
than unity, as probably seeing fewer spots than Wol: 
himself would reach with his instrument. These rel 
tive numbers, as tested by the most recent photographic^ 
results of De La Rue and Stewart, are found to be quite 
approximately proportional to the area covered by th 
spots. 

We give on the opposite page a figure deduced fro 
the numbers, published by Wolf in 1877, in the " Me- 
moirs of the Royal Astronomical Society," and showing 
their course year by year since 1772. The continua- 
tion"^ of the curve to 1880 is from numbers subse- 
quently published by him in the astronomical periodi- 
cals. The horizontal divisions denote years, and the 
height of the curve at each point gives the " relative 
number" for the date in question. For example, in 
1870, about the middle of the year, the relative num- 
ber was 140, while early in 1879 it ran as low as 3. 

The dotted lines are curves of magnetic disturbance, 
with which at present we have no concern. Our dia- 
gram, on account of the smallness of the page, only goes 
back to 1772, but Wolf's investigations reach to 1610, 
and he gives, in the paper from which were derived the 
numbers used in constructing our diagram, the follow- 
ing important table of the maxima and minima of sun- 
spots since that date, dividing the results into two series, 
the first of which, from the paucity of observations, is 
to be considered of much inferior weight to the second. 

* It did not seem worth while to re-engrave the plate in order to bring 
the curve down to date, but the main results since 1880 are stated nu- 
merically a page or two later. 



PERIODICITY OF SUN-SPOTS. 



156 



FiEST Seeies. 


Second Series. 


Minima. 


■ 
Maxima. 


Minima. 


Maxima. 


1610-8 




1615-5 




1745-0 


1750-3 




8-2 




10.5 


10-2 


11-2 


1619-0 




1626-0 




1755-2 


1761-5 




15-0 




13-5 


11-3 


8-2 


1634-0 




1639-5 




1766-5 


1769 7 




11-0 




9-5 


9-0 


8-7 


1645-0 




1649-0 




1775-5 


1778-4 




10-0 




11-0 


92 


9-7 


1655-0 




1660-0 




1784-7 


1788-1 




11-0 




15-0 


13-6 


16-1 


1666-0 




1675-0 




1798-3 


1804-2 




13-5 




10-0 


12-3 


12-2 


1679-5 




16850 




1810-6 


1816-4 




10-0 




8-0 


12-7 


13-5 


1689-5 




1G93-0 




1823-3 


1829-9 




8-5 




12-5 


10-6 


7-3 


1698-0 




1705-5 




1833-9 


1837-2 




14 




12-7 ! 


96 


10-9 


1712-0 




1718-2 


! 


1843 5 


1S48-1 




11-5 




9-3 


12-5 


12 


1723-5 




1727-5 




1856-0 


18601 




10-5 




11-2 


11-2 


10-5 


1734-0 




1738-7 




1867-2 


1870-6 


Mean period. 


Mean period. 


Mean period. 


Mean period. 


11-20 ± 2-11* 


11-20 ± 2-06 


11-16 ± 1-54 


10-94 ± 2-52 




±0-64 


±0-63 


±0-47 


±0-76 



From these data. Wolf derives a mean period of 
I 11-111 years, with an average variability of 2*03 years, 
j and an uncertainty of 0'30Y, due chiefly to the difliculty 
of fixing the precise date of maximum or minimum. 

After the great maximum of 1871*6, when the rel- 
ative number reached 140, there was an unusually pro- 
tracted down-slide until 1879, when, as the figure shows, 

*The upper number, ± 2-11, indicates that the individual periods 
have an average variation of 2-11 years on one side or the other from the 
mean period. The lower number, db 0-64, is the so-called " probable 
error " of the period. Similarly in the three other columns. 



156 THE SUN. 






a very low minimum occurred. After that a feeble 
maximum (only 64) arrived pretty quickly near tlie end 
of 1883, followed by an average minimum in tlie middle 
of 1889. The next and last maximum was passed in 
1893 ; it was not a very high one, perhaps about YO — 
but Wolf died in 1893, and we have no authentic figures 
later than 1891. 

A moment's inspection of the curve shows that the 
maxima differ greatly in intensity, and that the period 
is not at all fixed and certain like that of an orbital mo- 
tion, but is subject to great variations. Thus, between 
the maxima of 1829*9 and 1837*2 we have an interval 
of only 7*3 years, while between 1788 and 1804 it was 
16*1 years.^ A portion of this great variableness of 
period may, perhaps, be due to the incompleteness of 
our observations, but only a portion. It is quite likely 
that a fluctuation of much longer period, not far from 
sixty years, is, to some extent, responsible for the effect 
by its superposition upon the principal (eleven-year 
oscillation. 

Another important fact is that the interval from 
minimum to the next following maximum is only about 
4|- years on the average, while from the maximum to 
the next following minimum the interval is 6*6 years. 
The disturbance which produces the sun-spots springs 
up suddenly, but dies away gradually. 

Still another fact, as yet unexplained, and probably 
of great theoretical importance, has recently been 
brought out by Spoerer. Speaking broadly, the dis- 
turbance which produces the spots of a given sun-spoi 
period first manifests itself in two belts about 30^ north' 



,1! 



* Some astronomers contend that there ought to be another maximum 
inserted about 1795. Observations about this time are few in number 
and not very satisfactory. 



PERIODICITY OF SUN-SPOTS. 



157 



and south of the sun's equator. These belts then draw 
in toward the equator, and the sun-spot maximum occurs 
when their latitude is about 16° ; while the disturbance 
gradually and finally dies out at a latitude of 8° or 10°, 
some twelve or fourteen years after its first outbreak. 
Two or three years before this disappearance, however, 
two new zones of disturbance show themselves. Thus, 
at the sun-spot minimum there are four well-marked 
spot-belts ; two near the equator, due to the expiring 
disturbance, and two in high latitudes, due to the newly 
beginning outbreak ; and it appears that the true sun- 
spot cycle is from twelve to fourteen years long, each be- 
ginning in high latitudes before the preceding one has 
expired near the equator. 

Fig. 40 illustrates this, embodying Spoerer's results 
from 1855 until 1880. The dotted curves show Wolfs 



























Fig. 


40 


























18 


55 18 


60 18 


65 18 


70 18 


75 18 


80 


L. 

30 
26° 
22° 

18° 
14° 
10° 
























\^ 








/\ 


\ 


















W. 

100 

50 





\ 


\ 


< 


/' 


^^ 


\ 










\ 


\ 


Si 


s, 








\^ 














. 


^ 


/'' 




X 


X 


^ 


^ 


^ 




'N 


;i 


::zi; 


/ 




^ 


^ 


■^ 


- 


\ 


"^-. 


;::: 




^^ 


/ 



SPOERER'S CURVES OF SUN-SPOT LATITUDE. 



sun-spot curve for that period, the vertical column at 
the right of the figure, marked W at the top, giving 
Wolf's " relative numhers^^ The two continuous curves, 
on the other hand, give the solar latitudes of the two 
series of spots that invaded the sun's surface in those 
years. The scale of latitttdes is on the left hand. The 
first series began in 1856 and ended in 1868 ; the second 
broke out in 1866 and lasted until 1880. During these 



158 THE SUX. 

years it happened that there was very little difference 
between the northern and southern hemispheres of 
the sun. 

EXPLANATIONS OF SUN-SPOT PERIODICITY. 

There is no question of solar physics more interest 
ing or important than that which concerns the cause of 
this periodicity, but a satisfactory solution remains to 
be found. It has been supposed by astronomers of 
very great authority that the influence of the planets 
in some way produces it. Jupiter, Yenus, and Mer- 
cury have been especially suspected of complicity in 
the matter, the first on account of his enormous mass, 
the others on account of their proximity. De La Rue 
and Stewart deduced from their photographic observa- 
tions of sun-spots, between 1862 and 1866, a series of 
numbers, strongly tending to prove that, when two of 
the powerful planets are nearly in line as seen from 
the sun, then the spotted area is much increased. They 
have investigated especially the combined effect of Mer- 
cury and Venus, Jupiter and Yenus, and Jupiter and 
Mercury, as also the effect of Mercury's approach to, 
or recession from, the sun. In all four cases there 
seems to be a somewhat regular progression of num- 
bers, though much less decided in the third and fourth 
than in the first and second. The irregular variations 
of the numbers are, however, so large, and the duration 
of the observations so short, that it is hardly safe to 
build heavily upon the observed coincidences, since they 
may be merely accidental. In fact, so far as we can 
learn, the observations since 1866 furnish no confirma- 
tion of this theory. 

An attempt to connect the eleven-year period with 
that of the planet Jupiter also breaks down. While, 



PERIODICITY OF SUN-SPOTS. 159 

for a certain portion of time, there is a pretty good 
agreement between the sun-spot curve and that which 
represents the varying distance of Jupiter from the sun, 
there is complete discordance elsewhere. About 1870 
the maximum spottedness occurred w^hen the planet was 
nearest the sun, but at the beginning of the century the 
reverse was the case. Loomis suggested that the con- 
junctions and oppositions of Jupiter and Saturn may 
be at the bottom of the matter. These occur at inter- 
vals of 9*93 years, from a conjunction to an opposition, 
or vice versa. But, when we come to test the matter, 
we find that, in some cases, sun-spot minima have coin- 
cided with this alignment of the two planets ; in other 
cases, maxima. 

It is, indeed, very difficult to conceive in what man- 
ner the planets, so small and so remote, can possibly 
produce such profound and extensive disturbances on 
the sun. It is hardly possible that their gravitation 
can be the agent, since the tide-raising power of Venus 
upon the solar surface would be only about -yi^ of that 
which the sun exerts upon the earth ; and in the case of 
Mercury and Jupiter the effect would be still less, or 
about i-qVo" ^f ^1^^ sun's influence on the earth. Making 
all allowances for the rarity of the materials which com- 
pose the photosphere, it is quite evident that no planet- 
lifted tides can directly account for the phenomena. 
If the sun-spots are due in any way to planetary action, 
this action must be an occasion rather than a cause, A 
minute disturbance may, so to speak, " pull the trigger " 
and bring on an explosion. The touch of a child's 
finger fired the Flood Eock mine. 

Several astronomers, among others Professor B. 
Peirce, seem to have adopted an idea before alluded 
to — first suggested, we believe, by Sir John Herschel — 



160 THE SUN. 

that the spots are caused by meteors falling upon the 
sun. According to this view, the periodicity of the 
spots could be simply accounted for by supposing the 
meteors to move in a very elongated orbit, with a pe- 
riod of ll'l years, adding the additional hypothesis 
that at one part of the orbit they form a flock of great 
density, while elsewhere they are sparsely distributed. 
This meteoric orbit would have to lie nearly in the 
plane of the sun's equator, and have its aphelion near 
the orbit of Saturn. Of course there is no necessity to 
limit our hypothesis to a single meteor-stream. What 
we know of meteor-showers encountered by the earth, 
makes it likely that there may be several, of different 
periods ; and thus we may account for some of the ob- 
served irregularities of the sun-spot period. The hy- 
pothesis has many excellent points, and we shall have 
occasion to recur to it again. At the same time, it may 
be said here that it seems very difficult to make it ex- 
plain the enormous dimensions and persistence of many 
sun-spot groups, and the distribution of the spots on 
the sun's surface in two parallel zones, with a minimum 
at the equator. The irregularity in the epochs of max- 
ima and minima is also much greater than would have 
been expected. 

On the whole, it seems rather more probable that 
the periodicity is in the sun itself, depending upon no 
external causes, but upon the constitution of the photo- 
sphere and the rate at which the sun is losing heat. 
Perhaps we may compare small things w^ith great by 
referring to the periodic explosions of the Icelandic 
geysers, or the " bumping " of ether and many other 
liquids in a chemist's test-tube. Looking at it in this 
light, we should imagine the course of events to consist 
of a gathering of deep-lying forces during a season of 



PERIODICITY OF SUN-SPOTS. 161 

external quiescence, followed by an outburst, which 
relieves the internal fury ; the rest and the paroxysms 
recurring, at somewhat regular intervals, simply because 
the forces, materials, and conditions involved, change 
only slow^ly with the lapse of time. 

If such be really the case, it is clear, of course, that 
this periodicity is never likely to be very regular, and 
wnll not long keep step with any planetary march. 
Time of itself, therefore, will by-and-by solve the prob- 
lem for us, or at least will refute any false hypothesis 
resting upon the recurrence of planetary positions. 

TERRESTRIAL INFLUENCE OF SUN-SPOTS. 

Even more important than the problem of the cause 
of sun-spot periodicity, is the question whether this pe- 
riodicity produces any notable effects upon the earth, 
and, if so, what ? In regard to this question the astro- 
nomical world is divided into two almost hostile camps, 
so decided is the difference of opinion, and so sharp the 
discussion. One party holds that the state of the sun's 
surface is a determining factor in our terrestrial meteor- 
ology, making itself felt in our temperature, barometric 
pressure, rainfall, cyclones, crops, and even our financial 
condition, and that, therefore, the most careful watch 
should be kept upon the sun for economic as well as 
scientific reasons. 

The other party contends that there is, and can be, 
no sensible influence upon the earth produced by such 
slight variations in the solar light and heat, though, of 
course, they all admit the connection between sun-spots 
and the condition of the earth's magnetic elements. It 
seems pretty clear that we are not in a position yet to 
decide the question either way; it will take a much 
longer period of observation, and observations con- 

12 



I 



162 THE SUN. 

ducted with special reference to the subject of inquiry, 
to settle it. At any rate, from the data now in our pos- 
session, men of great ability and laborious industry 
draw opposite conclusions. 

It certainly is not so plain that the sun-spots have 
not the influence which their worshipers, I had almost 
called them, claim for them, as to absolve us from 
the duty of investigating the matter in the most thor- 
ough manner. On the other hand, it is also by no 
means certain that we shall find the labor of investiga- 
tion fruitful in precisely the manner and degree desired. 
Those who search for truth with honest endeavor may, 
nevertheless, be sure of their reward in some way. 

I have said that there is no doubt as to the con- 
nection between the sun-spots and terrestrial magnetism. 

In 1850, Lamont, of Munich, called attention to the 
fact that the average daily excursions of the magnetic 
needle have a period which, from the few decades of | 
observation at his command, he fixed at ten and one 
third years. 

Perhaps a word of explanation is needed here. 
Every one knows that the compass-needle does not 
point exactly north, and its divergence from the true 
meridian is different in difl^erent places. On the At- 
lantic coast of the United States, for instance, the north 
pole of the magnet points west of north, and on the 
Pacific coast east of north. What is more : at any par- 
ticular place the direction of the needle is continually 
changing, these changes being like the changes in the 
temperature of the air, in part regular and predictable, ; 
and partly lawless, so far as we can see. 

One of the most noticeable of the regular magnetic 
changes is the so-called diurnal oscillation ; during the 
early part of the day, between sunrise and one or two 



MAGNETISM AND SUN-SPOTS. 163 

o'clock p. M., the north pole of the needle moves toward 
the west in these latitudes, returning to its mean position 
about 10 p. M., and remaining nearly stationary during 
the night. The extent of this oscillation in the United 
States is about 15^ of arc in summer, and not quite half 
as much in winter ; but it differs very much in different 
localities and at different times, and also — and this is 
Lamont's discovery — the average extent of this diurnal 
oscillation at any given observatory increases and de- 
creases pretty regularly during a period of 10-J- years, 
according to his calculations. As soon as Schwabe an- 
nounced his discovery of the periodicity of the solar 
spots, Sabine in England, Gautier in France, and Wolf 
in Switzerland, at once and independently perceived the 
coincidence between the spot-maxima and those of the 
magnetic oscillation. Faye at one time attempted to 
impugn this conclusion. In order to make his point, he 
insisted that the magnetic maximum is shown by Cas- 
sini's observations to have occurred early in 1Y87, and, 
dividing the interval between this and the last magnetic 
maximum, near the close of 1870, by 8, the number of 
intervening periods, he gets 10'45 years for the mean 
magnetic period, instead of 11*11. The reply is, that 
the observations both of the sun-spots and of the mag- 
netic elements near the close of the eighteenth century 
are so meager and unsatisfactory that the evidence as 
to the precise time of maxima and minima is very in- 
complete. In 1885, however, Faye yielded to the con- 
stantly accumulating weight of evidence, and gave in 
his adhesion to the received conclusion, which is now 
practically undisputed. 

The convincing evidence as to the reality of the as- 
serted connection lies in the closeness with which, ever 
since we have been in possession of continuous and sat- 



164 THE SUN. 

isfactory observations, the magnetic curve copies that of 
the sun-spots. In Fig. 39 the dotted curves represent 
the mean amount of magnetic oscillation as deduced by 
Wolf from various series of observations. From 1820 
to 1895 the record is almost continuous, and the coinci- 
dence of the curves is such as to make it impossible to 
doubt the connection.^ 

The argument is much strengthened by an examina- 
tion of records of the aurora borealis. Occasionally so- 
called '' magnetic storms " occur, during which the com- 
pass-needle is sometimes almost wild with excitement, 
oscillating 5^ or even 10° within an hour or two. These 
" storms " are generally accompanied by an aurora, and 
an aurora is always accompanied by magnetic disturb- 
ance. 

ITow, when we come to collate aurora observations 
with those of sun-spots, as Loomis has done with great 
care and thoroughness, we find an almost perfect paral- 
lelism between the curves of auroral and sun-spot fre- 
quency. 

We find also, as Shearman, of Toronto, and Dr. 
Veeder, of Lyons, N. Y., have pointed out, that auroras 
often run in series, so to speak, following each other for 
several months at nearly regular intervals of 27*275 
days, which is very closely the period of the sun's ap- 
parent equatorial (synodic) rotation ; this of course 
makes it more or less probable that their appearance is 
connected somehow with the way in which certain por- 
tions of the sun's surface present themselves to the 
earth. Dr. Yeeder's idea is that disturbed regions upon 

* A discussion, by Balfour Stewart, of the observations at Kew, be- 
tween 1856 and 1867, brings out the correspondence very beautifully, 
and seems to show that the magnetic changes lag behind the sun-spots 
about five months. 



II 



MAGNETISM AND SUN-SPOTS. 165 

the sun are specially influential upon tlie earth's mag- 
netism at the moment when they are near the eastern 
edge of the sun, and just coming in sight to us on the 
earth. There is no obvious reason, however, why a 
disturbance on the sun should thus propagate itself more 
vigorously in a direction tangential to the sun's surface 
and in the plane of the sun's equator than in any 
other direction ; and while Dr. Veeder is certainly able 
to marshal a great number of coincidences in support 
of his opinion, there are also numerous cases where the 
region of solar disturbance was near the middle of the 
sun's disk, as, for instance, the great magnetic storms and 
auroras of February 13, 1892, and I^Tovember 17, 18S2. 
In this connection it seems worth while to quote from 
an article by Mr. Maunder, of the Greenwich Observa- 
tory, with respect to these spots, and two other groups 
of almost equal magnitude which appeared together in 
April, 1882. He writes : 

*^ In a period of nearly nineteen years, therefore " (from 1873 
to 1892), '* we have three magnetic storms which stand out pre- 
eminently above all others during that interval. In that same 
period we have three great sun-spot displays — counting the two 
groups of April, 1882, together — which stand out with equal dis- 
tinctness far above all other similar displays. And we find that 
the three magnetic storms were simultaneous with the greatest de- 
velopment of the spots. Is there any escape from the conclusion 
that the two have a real and binding connection? It may be di- 
rect ; it may be indirect and secondary only ; but it must be real 
and effective."— "Knowledge," May, 1892. 

It is not easy to frame any satisfactory theory to ac- 
count for this connection between solar disturbances and 
terrestrial magnetism. It can hardly be in the way of 
temperature, for the influence of sun-spots in this re- 
spect is so slight that it is still an open question whether 
we do or do not get from the sun more than the average 



166 THE SUN. 

amount of heat during a sun-spot maximum. Probably 
it is more immediate and direct ; perhaps in some way 
kindred with the action which drives off the material of 
a comet's tail, and proves that other forces besides grav- 
itation are operative in interplanetary space. Or, not 
at all impossibly, as Mr. Maunder suggests, it may be 
indirect ; an action of some cosmic cause upon sun and 
earth together. 

There are a number of observed instances which, 
though not sufficient to demonstrate the fact, still ren- 
der it very probable that every intense disturbance of 
the solar surface is propagated to our terrestrial mag- 
netism with the speed of light. An instance fell under 
the writer's notice in the course of a series of spectro- 
scopic observations at Sherman. On August 3, 1872, 
the chromosphere in the neighborhood of a sun-spot, 
which was just coming into view around the edge of the 
sun, was greatly disturbed on several occasions during 
the forenoon. Jets of luminous matter of intense bril- 
liance were projected, and the dark lines of the spectrum 
were reversed by hundreds for a few minutes at a time. 
There were three especially notable paroxysms at 8.45, 
10.30, and 11.50 a. m. local time. At dinner the pho- 
tographer of the party, who was determining the mag- 
netic constants of our station, told me, without knowing 
anything about my observations, that he had been obliged 
to give up work, his magnet having swung clear off the 
scale. Two days later the spot had come around the 
edge of the limb. On the morning of August 5th I 
began observations at 6.40, and for about an hour wit- 
nessed some of the most remarkable phenomena I have 
ever seen. The hydrogen lines, with many others, were 
brilliantly reversed in the spectrum of the nucleus, and 
at one point in the penumbra the C line sent ont what 



MAGNETISM AND SUX-SPOTS. 



167 



looked like a blowpipe-jet, projecting toward the up- 
per end of the spectrum, and indicating a motion along 
the line of sight of about one hundred and twenty miles 
per second. This motion would die out and be renewed 
again at intervals of a minute or two. The figure gives 
an idea of the appearance of the spectrum. The dis- 
turbance ceased before eight o'clock and was not re- 
newed that forenoon. On writing to England, I re- 



FiG. 41. 



/7 7r Y ^ 


c C ^s 

nhliiN mill 


68' G7 6 6 GJ gL (S j 

iilillilliillillilliii liliiillilililhili ;iilliii! h!! 


1 


iiiiiiiiiM 
























^I^^^M 






1 




i 


1 


|j||r;: 


1 


1 


1 


1 


1 


1 


iliiiiiiiiBaiif!i:kii!: 


■III 






^ 




1 


:||P*'^: 


1 

1 


















"\mm 


m 


1 


C LINE IN Spot Spectrum. Aug. 5,1872. 



ceived from Greenwich and Stonyhurst, through the 
kindness of Sir G. B. Airy and Rev. S. J. Perry, copies 
of the photographic magnetic records for those two 
days. Fig. 42 is reduced from the Greenwich curve. 
That obtained at Stonyhurst is essentially the same. It 
will be seen that on August 3d, which was a day of gen- 
eral magnetic disturbance, the three paroxysms I noticed 
at Sherman were accompanied by peculiar twitches of the 
magnets in England. Again, August 5th was a quiet 
day, magnetically speaking, but just during that hour 
when the sun-spot was active, the magnet shivered and 



IQg THE SUN. 

trembled. So far as appears, too, tlie magnetic action 
of the sun was instantaneous. After making allowance 
for longitude, the magnetic disturbance in England ap- 
pears strictly simultaneous, so far as can be judged, 

Fig. 42. 



gMj^—I^M^M^Bl— —I— —^IW^^—^^lBM^IMl^—^^WiMiW— 



Magnetic Curves at Greejswich (August 3 and 5, 1872). 

with the spectroscopic disturbance seen on the Eocky 
Mountains, and the difference can not have been more 
than about ten minutes. But the time at Sherman 
w^as not noted with any great precision. 

Of course, as has been said, no two or three coinci- 
dences such as have been adduced are sufficient to es- 
tablish the doctrine of the sun's immediate magnetic 
action upon the earth, but they make it so far probable 
as to warrant a careful investigation of the matter — an 
investigation, however, which is not easy, since it im- 
plies a practically continuous watch of the solar surface. 



II 



MAGNETISM AND SUN-SPOTS. 169 

It may be added, too, that many striking disturb- 
ances which have been observed npon the snn, in the 
ascent of lofty prominences, received no magnetic re- 
sponse from the earth ; and there have also been great 
auroras with no obvious solar correlative. Indeed, 
there is every reason to suppose that a large proportion 
of all the magnetic disturbances at any given observa- 
tory are purely local, having nothing whatever to do 
with the sun. Some also which are not local have 
been traced to the action of the moon, and it is not at 
all improbable that others yet are due to causes oper- 
ating in interplanetary space. 

Solar disturbances are not the cause of our maprnetic 
storms, but only one cause of some of them ; and very 
likely a cause only in the sense that the pulling of a 
trigger "causes" the flight of a rifle-ball : there need be 
no proportionality between such a cause and its effect. 

It would be unfair to our readers to pass without 
notice the remarks of Lord Kelvin in a recent presiden- 
tial address to the Royal Society (November, 1892), ex- 
pressing his dissent from the accepted view of the 
relation we have been discussing. Taking the magnetic 
storm of June 25, 1885, as an example, he computes that 

" In this eight hours of a not very severe magnetic storm as much 
work must Lave been done by the sun in sending magnetic waves 
out in all directions through space as he actually does in four 
months of his regular heat and light. This result," he adds, "it 
seems to me, is absolutely conclusive against the supposition that 
terrestrial magnetic storms are due to magnetic action of the 
sun, or to any kind of action takiug place within the sun, or in 
connection with hurricanes in his atmosphere, or anywhere near 
the sun outside. It seems as if we may also be forced to con- 
clude that the supposed connection between magnetic storms and 
sun-spots is unreal, and that the seeming agreement between the 
periods has been a mere coincidence." 



II 



170 THE SUN. 

And yet, with all deference to so liigh an authority, 
and without questioning the accuracy of his calculations, 
they seem really to be no more conclusive than a com- 
putation to show that the work done by an explosion 
vastly exceeded the power of the person who pressed 
the firing button. The nature of the mechanism by 
which the connection is established may, and still 
does, remain uncertain, but the statistics leave no doubt 
as to the reahty of the connection itself. It is not, per- 
haps, outside the limits of possibility, as before hinted, 
that both the solar and terrestrial disturbance have a 
common origin in some invasion of power or matteri 
from outer space — that the solar tumult is the brother 
and not the father of our own aurora. 

As to the effect of sun-spots upon terrestrial temper- . . 
ature, no conclusion seems possible at present. The|| 
spots themselves, as Henry, Secchi, Langley, and others 
have shown, certainly radiate to us less heat than the 
general surface of the sun. According to the elaborate .. 
determinations of Langley, the umbra of a spot emits |j 
about fifty-four"^ per cent, and the penumbra about 
eighty per cent, as much heat as a corresponding area 
of the photosphere. The direct effect of sun-spots is, 
therefore, to make the earth cooler. As the total area 
covered by spots, even at the time of maximum, never 
exceeds -^-^ of the whole surface of the sun, it follows 

* The most recent observations, those made at Daramona, in Ireland, ] 
by W. E. Wilson, in 1893, with a " radio-micrometer " and other apparatus * 
of the highest order, give about forty-six per cent, for this ratio. All ob- 
servers find that it increases near the limb of the sun, and both Langley 
and Frost have encountered cases where the umbra of a spot was appar- 
ently warmer than the surrounding photosphere : a fact which, if not the 
result of some error of observation, is difficult to explain on the theory 
that spots arc cavities, though a necessary consequence if they, like the 
faculaD, arc masses floating at some elevation above the photosphere. 



MAGNETISM AND SUN-SPOTS. 171 

that directly thej may diminish our heat-snpply by 
about YoVo^ ^f "^^ whole. Whether this effect would 
be sensible or not, is a question not easily answered. 

But, while the direct effect would be of this nature, 
it is quite probable that it is at least fully compensated 
by another of the opposite character. We get our hght 
and heat from the photosphere which is covered by an 
atmosphere of gases, and in this atmosphere a consider- 
able absorption occurs. Now, if the level of the photo- 
spheric surface be disturbed, so that it is covered with 
waves and elevations of any considerable height, as 
compared with the thickness of the overlying atmos- 
phere, then, as Langley has shown, the radiation will 
at once be increased ; since, while the absorption is in- 
creased by a certain percentage for those portions of 
the photosphere which are depressed below their ordi- 
nary level, it is much more decreased for those that are 
raised. 

The reason of this is that, when a luminous object 
is immersed in an absorbing medium it loses much 
more light for the first foot of submergence than for 
the second, and more for the second than for the third ; 
i so that when it has reached a considerable depth it re- 
I quires an additional submergence of many feet to di- 
I minish its radiation as much as the first foot did. If, 
I therefore, sun-spots are accompanied by considerable 
I vertical disturbance of the photosphere, as is almost 
certain, we must have as a result an increased radia- 
tion on account of the disturbance, offsetting, more or 
less entirely, the opposite effect which is at first view 
i most obvious. 

I Then, again, it is altogether probable that spots are 
either due to, or accompanied by, an eruptive action — 
the internal, and hotter, gases bursting through the pho- 



l-fl 



172 THE SUN. 

tospliere witli unusual abundance during seasons of 
spot-maximum. This must necessarily tend to increase 
the emission of heat from the sun, and possibly by a 
considerable amount. But, on the other hand, any 
considerable increase in the thickness of the chromo- 
sphere, such as might result from abundant and long- 
continued eruption, would work in the opposite direc- 
tion. 

It is impossible, therefore, to predict, a priori^ which 
effect will predominate, or to say whether the mea 
temperature of the earth ought to be raised or lowered] 
during a sun-spot maximum ; and thus far no compariJ 
son of observations has settled the matter to general 
satisfaction. At least, no longer ago than 1878, Balfour 
Stewart, w^ho ought to know if any one, writes, " It is 
nearly, if not absolutely, impossible, from the observa- 
tions already made, to tell whether the sun be hotter or 
colder, as a whole, when there are most spots on his 
surface." 

On the one hand, Jelinek, from all temperature 
observations available in Germany up to 1870, found 
the influence of sun-spots entirely inappreciable, though 
from the same observations he did deduce minute effects 
produced by the changes in the distance and phase of 
the moon. On the other hand, Mr. Stone, while astrono- 
mer royal at the Cape of Good Hope, and Dr. Gould, in 
South America, consider that the observations taken at 
their stations show a distinct though slight diminution 
of temperature at the time of a sun-spot maximum ) 
according to Dr. Gould the difference at Buenos Ayres 
between maximum and minimum amounts to about 
lf° Fahr. He also considers that the meteorological 
records of the Argentine Republic between 1875 and 
1885 show a distinct connection between the sun-spots 



METEOROLOGY AND SUN-SPOTS. 173 

and the force and direction of the winds at the various 
stations. At the Cape of Good Hope, Mr. Stone finds 
the difference to be about three fourths of a degree 
from thirty years' observations — at least, if we rightly 
interpret his curve of temperatures, for it is not quite 
clear what unit of temperature is used in constructing 
his diagram. 

At Edinburgh, Piazzi Smyth finds in the records 
of the rock thermometers a marked eleven-year perio- 
dicity, of which the range amounts to about a degree 
(Fahr.), and the maxima, instead of coinciding with the 
sun-spot minima, come about two years behind them. 

On the whole, perhaps, as things now stand, it would 
be fair to say that there is a «mall balance of probability 
in favor of the statement that years of sun-spot maxi- 
mum are a degree or so cooler than those of spot-mini- 
mum ; but the balance is very slight indeed, and the 
next investigation of somebody else may carry it to the 
other side. 

As regards the influence of sun-spots upon storms 
and rainfall, the evidence, if not entirely conclusive, as 
it is considered by Mr. Lockyer and some other high 
authorities, is at least considerably stronger. In 1872 
Mr. Meldrum, director of the observatory at the Mau- 
ritius, published a comparison between the number of 
cyclones observed in the Indian Ocean and the state of 
the sun, and pointed out that the number of cyclones 
was greatest at the time of a sun-spot maximum. We 
quote his words {" Nature," vol. vi, p. 358) : " Taking 
the maxima and minima epochs of the sun-spot period, 
and one year on each side of them, and comparing the 
number of cyclones in these three-year periods, we get 
the following results : 



174 



THE SUN. 



Years. 


No. of cyclones 
in each year. 


Total No. of 
cyclones. 




( 1847 


4) 
6 V 
6 ) 
4 ) 
1 [ 

3 ) 
5 

i\ 
l\ 
l\ 




Maxima . . 


\ 1848 


15 




( 1849 






( 1855 




Minima. . . 


\ 1856 


8 




( 185Y 






i 1859 




Maxima . . 


. - 1860 

( 1861 


21 




C 1866 




Minima. . . 


. } 1867 


9 




( 1868 






( 1870.. 




Maxima . . 


. -| 1871 


14" 




( 1872 











Subsequently Mr. Meldrum made more extensive 
comparisons, including not only cyclones proper, but 
other great storms, and brings out essentially the same 
results. At the same time it is to be noted that the 
yearly numbers vary enormously, and, on referring to 
his second paper (" Mature," vol. viii, p. 495), it will be 
found that the number for the sun-spot maximum, 
1847-'49, is only twenty -three, while that for the mini- 
mum, 1866-'68, is twenty-one. (Mr. Meldrum coaxes 
the first sun-spot maximum a little by using the years 
1848-'50 in his comparison ; rather unwarrantably, it 
would seem, since the epoch of spot maximum was 
1848-1 : by using those years, he gets twenty-six instead 
of twenty-three.) 

The variations from year to year are so extreme that 
it is sufticient to say that the observations can hardly be 
considered as demonstrative without much further con- 
firmation from other sources. 

Mr. Meldrum has attempted to supply this confirma- 
tion by tabulating the rainfall at a number of stations 



METEOROLOGY AND SUN-SPOTS. 175 

in and near the Indian Ocean, and obtains a result con- 
firmatory on tlie whole, though there are several discrep- 
ancies. Mr. Lockyer, from observations of the rainfall 
at the Cape of Good Hope and Madras, gets corrobora- 
tive figures. 

Mr. Meldrum, in a still later paper published in the 
" Monthly Notices of the Mauritius Meteorological So- 
ciety," for December, 1878, discusses at length the 
rainfall of more than fifty different stations in all parts 
of the earth, and also the levels of many of the princi- 
pal European rivers. The discussion covers nearly all 
the available data from 1824 to 1867. It is only just 
to Mr. Meldrum to say that the treatment seems to be 
sufficiently thorough, perfectly fair, and the result on 
the whole is in favor of his opinion that there is a real 
connection between the annual rainfall and the state of 
the solar surface. He finds the average rainfall for the 
earth to be about 38'5 inches annually ; the range be- 
tween the maximum and minimum is about four inches ; 
and the rainfall maximum occurs about a year after the 
sun-spot maximum, though with a good deal of varia- 
tion at different stations. In some countries, indeed, 
and at some times (in the United States, for instance, 
between 1834 and 1843), the results conflict with the 
theory, but the general accordance is striking, and seems 
to warrant his concluding statement that " the mean 
rainfalls of Great Britain, the Continent of Europe, 
America, and India, as represented by all the returns 
that have been received, have, notwithstanding anom- 
alies, varied directly as Wolf's sun-spot numbers have 
varied, and the epochs of maximum and minimum rain 
have nearly coincided with those of the sun-spots. The 
rainfalls at five stations in the southern hemisphere, for 
shorter periods, give similar results." 



176 THE SUN. 

Mr. Symons, from the British rainfall of the past 
one hundred and forty years, gets an equivocal result. 
American stations, so far as they have been tested, are 
on the whole rather in opposition to those of the Indian 
Ocean, indicating somewhat less rain than usual during 
a sun-spot maximum. But, as any one can see by con- 
sulting Mr. Symons's paper in ''Nature," vol. vii, pp. 
143-145, in which he has tabulated an immense number 
of rainfall statistics, the evidence is extremely conflict- 
ing — altogether different in force and character from 
that which demonstrates the magnetic influence of solar 
disturbances. 

Still other attempts have been made to establish a 
connection between sun-spots and various terrestrial 
phenomena. Thus, Dr. T. Moffat, in 1874, published 
results tending to show that in sun-spot years the aver- 
age quantity of atmospheric ozone is somewhat greater 
than during a spot-minimum. 

Another eminent physician, whose name escapes us, 
endeavored, some years ago, to show that the visitations 
of Asiatic cholera are periodical, and that their period 
depends upon that of the sun-spots, being just once and 
a half as long — about fifteen years. This periodicity 
may be ve^^^ perhaps j but, if so, the fact that the chol- 
era maxima are alternately sjmchronous with the max- 
ima and minima of the spots, w^ould be sufficient to 
disprove the idea of any casual connection between the 
phenomena. 

One of the most interesting of the essays in this 
direction, is that of Professor Jevons, who sought to 
sliow a relation between sun-spots and commercial 
crises. The idea is by no means absurd, as some havoi 
declared — it is a mere question of fact. If sun-spots 
have really any sensible effect upon terrestrial meteor- 



INFLUENCE OF SUN-SPOTS. 177 

ology, upon temperature, storms, and rainfall, they must 
thus indirectly affect the crops, and so disturb financial 
relations ; in such a delicate organization as that of the 
world's commerce, it needs but a feather-weight, rightly 
applied, to alter the course of trade and credit, and pro- 
duce a " boom " (if we may be forgiven the use of so 
convenient a word) or a crash. 

We have not time or space to discuss Mr. Jevons's 
paper, but must content ourselves with saying that, to 
us at least, the facts do not seem fairly to warrant his 
conclusion. 

It can do no harm to reiterate and emphasize what 
was said a few pages back, that the question of sun-spot 
influence can not be considered settled ; and that the 
only method of deciding it is by a continuous series of 
careful observations, conducted specially for the pur- 
pose, or at least conducted with reference to the con- 
ditions of the problem, since the same observations 
would also be useful as data for various other investi- 
gations. 

While it is not at all unlikely that investigation 
will result in establishing some real influence of sun- 
spots upon our terrestrial meteorology and determin- 
ing its laws, it is practically certain that this influence 
is extremely slight, and so masked and veiled by other 
influences more powerful that it is extremely difii- 
cult to bring to light. 

SUN-SPOT THEORIES. 

Naturally, the remarkable phenomena of the sun- 
spots have invited speculation as to their cause. 

As has been mentioned already, some of the early 
observers believed the spots to be planetary bodies cir- 
culating around the sun, very near its surface. This 
13 



178 THE SUN. 

opinion Galileo unanswerably refuted by pointing out 
that in that case the spot, in its movement around the 
sun, ought to be visible less than half the time. He, 
on the other hand, proposed the theory that they are 
clouds, floating in the solar atmosphere. 

This view, in one form or another, has since been 
held by many astronomers of great authority. Derham 
believed these clouds to be eruptions from solar volca- 
noes, and in our own times Capocci has adopted and 
maintained the same theory. Peters seems to have con- 
sidered it favorably in 1846, at least so far as the vol- 
canic part of the hypothesis is concerned, while Kirch- 
hoff seems to have assented to Galileo's original opinion 
unmodified. If the statement be interpreted to mean 
that sun-spots are masses of cloudy matter, less luminous 
than the photosphere, and floating m, not above^ the 
photosphere, probably a very large proportion of the 
students of solar physics would to-day agree to it. Gal- 
ileo, however, believed the spot-clouds to be high above 
the shining surface, which we now know not to be the 
fact ; for the observations of Wilson, in 1769, men- 
tioned a few pages back, and the whole body of obser- 
vations since then, have made it almost certain that the 
umbra of a sun-spot lies several hundred miles below 
the level of the photosphere.^ 

Lalande, however, was not disposed to accept Wil-j 
son's doctrine, and maintained that the sun-spots are 
the tops of solar mountains projecting above the lumi- 
nous surface — islands in tlie ocean of fire. In this hy- 
pothesis the penumbra is accounted for by the shelving 
sides of the mountains seen through the semi-trans- 
parent flame. It will be noticed that the theories 

* But we must not overlook Mr. Hewlett's conclusions (p. 129), nor 
the observations of E. Wilson and Frost (note to p. 170). 



II 



SUN-SPOT THEORIES. 179 

already mentioned, as well as that of Sir William 
Herscliel, wliicli we must now present, all proceed 
upon the assumption that the central core of the sun 
is solid. 

About the beginning of the present century, Sir 
WilUam Herscliel, after a careful study of the facts, 
but much influenced by the belief that the sun must 
(for theological reasons) be a habitable body, proposed 
an hypothesis which stood unchallenged for nearly half 
a century. 



Fig. 43. 




photo-sphere:. 

PEINUMBRAL CLOUD. 
BODy OF SUN. 



W. Heuscbel's Sun-Spot Theoey. 



He supposed the central portion of the sun to be 
solid ; its surface cool, non-luminous, and habitable. 
Around this he placed two envelopes of cloud — the 
outer one, the photosphere, incandescent, blazing with 
unimaginable fury ; the inner one non-luminous, dark 
itself, but capable of reflecting light from its upper sur- 
face, and acting as a screen to protect the underlying 
country from the heat of the photosphere. The spots 
he supposed to be caused by temporary openings in 
the clouds, through which w^e could look down upon 
the dark surface of tlie central globe ; the penumbra 
being caused by the intermediate cloud-layer, opening 
less widely than the photosphere. The figure illustrates 
this theory. As to the cause of the openings he uttered 
no decided opinion, though suggesting that they might 
be due to volcanic eruptions, forcing their way up 
through the higher atmosphere. 

His son. Sir John Herscliel, many years later, pro- 






180 THE SUN. 

posed an explanation which would make the spots to be 
great whirling storms horing down through the photo- 
sphere and clouds, instead of eruptions pushing their 
way outward. According to him, the rotation of the 
sun causes an accumulation of the solar atmosphere at! 
the sun's equator — a thickening of the layer which ob- 
structs the radiation of heat. This being so, there should 
be on the sun, as on the earth, though for an entirely 
different reason, a temperature higher in the equatorial 
regions than elsewhere ; and then would follow a long 
train of consequences, among them these : the solar at 
mosphere would be disturbed by currents like the trade- 
winds on the earth ; there would be stormy zones on 
each side the equator, and these storms w^ould furnisl:^ 
an explanation of the spots. 

To a certain extent, the cause adduced must actually 
exist. The sun's rotation must necessarily thicken th< 
atmospheric layer which overlies the photosphere (i. e, 
it must, if the surfaces of the photosphere and chromo 
sphere can be regarded as level surfaces), and this causi 
must tend to raise the actual temperature of the sun'i 
equator, w^hile at the same time it must diminish its' 
radiation to the earth, and so render the solar equate^ 
apparently cooler^ as tested by our observations fron^ 
the earth. But, so far as can be judged, this effect is 
quite insensible, as it should be, since the sun's rotation 
is so slow ; and the motions of the spots show no such 
systematic drift north or south as solar trade-wnnds 
would necessarily produce. 

The elder Herschel's theory satisfies all the tele- 
scopic appearances of sun-spots quite as well, perhaps, 
as any yet proposed. It breaks down in its assumption 
that the principal portion of the sun is a solid mass, an 
assumption which is now almost universally regarded 



SUN-SPOT THEORIES. 



181 



as incompatible with what we know of the solar tem- 
perature, radiation, and constitution. 

It seems to modern physicists an unavoidable con- 
clusion that the sun's central mass must be gaseous, or 
at least not sohd. Setting out with this idea, Faye and 
Secchi independently, about 1868, proposed the theory 
that the spots are openings in the photosphere, through 

Fig. 44. 




SECcnf 9 FiEST Spot-Theokt. 



which the internal gases are bursting outward. We 
present one of Secchi's figures illustrating this view. 
But it was abandoned by its proposers as soon as it was 
clearly pointed out that in that case the spectrum of 
the umbra of a sun-spot should be composed of bright 
lines ; and Secchi himself and others had shown that it is 
not so at all, but a spectrum due to increased absorption, 



182 THE SUK 

and probably indicating, not an np-rush of heated gases 
through the photosphere, but a descent of cooler 
and less luminous matter from above. In this connec- 
tion the observations of the writer and of Duner may 
be referred to (page 132). But the theory has great 
vitality. Mr. Proctor, in his " Old and New Astron- 
omy," maintained it, and it continually turns up in 
the speculations of popular writers. About 1870 ZoU- 
ner proposed a peculiar theory which has many good 
points about it, but seems obnoxious to fatal objec- 
tions, and has found very few defenders. He conceives 
the surface of the sun to be liquid — a molten mass over- 
laid by an atmosphere of vapor. This liquid surface he 
imagines to be here and there covered at times by slag- 
like masses of much lower radiating power, the result 
of local cooling. Around their edges the solar flames 
burst out with redoubled fury, but at the center the 
cooler mass of scoria determines a downward current, 
so as to establish a powerful circulation in the solar at- 
mosphere — downward at the center of the spot, outward 
in all directions at the surface of the slag, upward all 
around its margin, and inward, toward the center, in 
the upper air. This theory admirably agrees with the 
spectroscopic phenomena ; but the hypothesis of a con- 
tinuous liquid shell, cool enough to permit the forma- 
tion of scoriae, seems inconsistent with other phenom- 
ena, which make it impossible to admit so low a tem- 
perature at so great a depth. 

At present, opinion, for the most part, seems to be 
divided between two rival theories proposed by Faye 
and Secchi. 

Faye conceives the sun-spots to be the effect of 
solar storms ; Secchi believes them to be dense clouds 
of eruption-products settling down into the photo- 



I 



SUN-SPOT THEORIES. 183 

sphere near^ but not at^ the points where they were 
ejected. 

Faye, it will be remembered, supposes the sun's pe- 
culiar law of rotation to be due to the hypothetical fact 
that the ascending masses of vapors (which form the 
photosphere by their condensation) start from a stratum 
whose depth below the visible surface regularly dimin- 
ishes from the equator toward the poles. Hence re- 
sult currents parallel to the equator, and the conse- 
quence is that, generally speaking, neighboring portions 
of the photosphere have a relative drift. At the equa- 
tor and at the poles this drift vanishes, but is most con- 
siderable in the middle latitudes. Now, it is Faye's 
theory that, in consequence of this relative drift, eddies 
are formed, as explained on a preceding page ; these 
eddies become cyclones or whirls precisely analogous 
to those seen in water where a rapid current is obstruct- 
ed by an obstacle. In such a case, as every one knows^ 
tunnel-shaped vortices are formed, down which floating 
materials and air are carried to considerable depths. 
Our terrestrial whirlwinds and tornadoes are produced, 
according to Faye (but in opposition to the generally 
received theories), in a similar manner, beginning from 
above^ and penetrating downward until the point of the 
whirling vortex reaches and sweeps the earth. Now, 
such a vortex, on the solar scale, is the essence of a sun- 
spot, according to Faye. 

It is evident at once that this theory gives a reason- 
able explanation of the distribution of the spots in two 
parallel zones on each side of the sun's equator, and that 
the drifting action, in which the cause of the spots is 
supposed to lie, is a vera causa. 

The theory accords very well, also, with the phe- 
nomena which accompany the subdivision of spots, 



184 THE SUN. 

since whirls in water and cyclones in the terrestrial 
atmosphere behave in precisely the same sort of way. 
It fairly meets, too, the spectroscopic indications. The 
cavity filled with descending vapors would naturally 
give just such a kind of spectrum as that which is ordi- 
narily observed. Moreover, the gases carried down in 
the vortex below the photosphere, especially the hydro- 
gen, would boil up again all around the whirlpool, and 
thus we could account for the ring of faculse and prom- 
inences which, as a general rule, environs every spot of 
considerable magnitude. Some of the more obvious 
objections can also be easily disposed of. Thus, it 
has been said that, if the sun-spots are such vortices, 
they ought to be circular in outline. Faye replies that 
we see, not the vortex itself, but a great cloud of cooler 
gases, sucked down from above and gathered into the 
storm from all sides, and the form of this cloud would 
depend upon a multitude of circumstances. 

But there are other objections which are not so easily 
met. It the theory be true, all spots are whirls and 
ought to show a vortical motion, and, what is more, all 
spots north of the equator ought to whirl in the same 
direction, and against the hands of a watch (as seen 
from the earth), while those in the sun's southern hemi- 
sphere should revolve in the contrary direction, pre- 
cisely as cyclones do in the atmosphere of the earth. 

Now, this is not the case at all. As we have seen, 
only a very small percentage of the spots show any 
trace of vorticose motion ; and, so far from observing 
any uniformity in the direction of rotation on each side 
of tlie equator, we frequently find different members of 
the same group of spots, or even different portions of 
the self-same spot, revolving oppositely. 

In fact, when we come to look into the matter nu- 



SUN-SPOT THEORIES. 185 

merically, we find tliat the drifts which Faye makes 
the determining factor of sun-spot genesis, is far too 
slight to produce such effects. 

It is very easy to compute this drift if we assume 
the correctness of Faye's own formula for the motion 
of a point on the sun's surface in any given solar lati- 
tude, viz., V ^ 862' — 186' sin' X; V in this formula 
being the number of minutes of solar longitude passed 
over by any given point in twenty-four hours. 

If we apply this formula to two points on the solar 
surface, one in latitude 20° and the other in latitude 
20° 1', i. e., about 123 miles north of the first, we shall 
find that the first has a daily motion of 840*24:2' and 
the second 840*207', a difference of only -035', or (in 
this latitude) 4*17 miles. That is to say, if we take two 
points on the solar surface, on the same meridian, in 
latitude 20°, at a distance of 123 miles, the one nearer 
the equator will, at the end of twenty-four hours, have 
drifted about 4^ miles to the eastward of the other. 

If we make the same calculation for latitude 45% 
we get a result a trifle greater — about ^ miles per day. 

With these figures it is easy to see why the sun-spots 
do not behave more like the disturbances of our terres- 
trial atmosphere, in exhibiting cyclonic motion as a 
regular and invariable characteristic, instead of an occa- 
sional and rather a rare phenomenon. 

Secchi's latest theory is based essentially upon the 
idea, certainly borne out by observation, that eruptions 
are continually breaking through the photosphere, and 
carrying up metallic vapors from the regions beneath. 
He imagines that these vapors, after becoming consid- 
erably cooled, descend upon the photosphere and form 
depressions in it, which are filled with these less lumi- 
nous and absorbent materials. It is difficult to see why 



18(3 THE SUK 

the effect should remain so persistent, or why^ even if 
the eruption be long maintained, the cloud should con- 
tinue to descend in the same place. In fact, as was 
said only a few moments ago, a spot is generally sur- 
rounded by a ring of eruptions, and things take place 
as if they were all pouring their ejections into the same 
receptacle — as if there were, in fact, some such down- 
ward suction through the center of the spot as the the- 
ory of Faye supposes, an aspiration capable of drawing 
in tow^ard the spot all erupted materials in the vicinity. 
The sun-spot theories of Lockyer and Schaeberle, 
already referred to on page 143 in connection with the 
explanation of the equatorial acceleration of the sun^s 
rotation, agree with this theory of Secchi's in attributing 
the spots to the downfall of matter from a great ele- 
vation. Schaeberle supposes it to be matter simply 
blown out by eruptions, some of it with force enough 
to carry it out even to the orbits of Jupiter and Saturn ; 
on its return it penetrates and chills the photosphere. 
Mr. Lockyer, if we understand him rightly, in his sug- 
gestions which form the closing chapter of the " Chem- 
istry of the Sun," is rather disposed to think that the 
" iron " and such other substances as by their fall pro- 
duce the spots are formed by the union and combina- 
tion of their elementary constituents, which have as- 
cended in a " dissociated " condition to the upper regions 
of the solar atmosphere. There, where the temperature 
is no longer above the '^ dissociation" point, the atoms 
recombine into molecules of iron-vapor, etc. ; the vapors 
condense into clouds and liquid masses, and these de- 
scend upon the photosphere. They absorb heat all the 
way down, by revaporization and new dissociation chill- 
ing the photosphere where they pierce it and causing a 
" splash " or up-rush of the photosphei-ic matter and its 



SUN-SPOT THEORIES. 187 

nnderlying gases all around the spot, which we recog- 
nize as faculse, prominences and metallic eruptions. 
According to these theories the facute and eruptions 
are coiisequences of the formation of the spot : accord- 
ing to Secchi i\\ej precede and cause it. It would seem 
easy to decide the question by observation, but it does 
not appear to be so ; on the whole, however, the weight 
of evidence is pretty strongly in favor of the opinion 
that faculse and pores and a general disturbance of the 
region are usually obvious before the spot manifests it- 
self ; and it must be admitted that in some cases the 
appearances puzzlingly resemble the emergence of a 
dark insL^sfroiii beneath. 

Probably both Lockyer and Schaeberle would cheer- 
fully accept Sir John Herschel's theory to a certain ex- 
tent — that some of the spots may be due to the fall 
upon the sun of great meteors from outer space. While 
it is hardly possible that, directly, a meteor, such as we 
know meteors upon the earth, could by its fall produce 
even a small sun-spot, it is not easy to say what might 
be the indirect effects consequent upon its passage 
through the photosphere, and its disturbance of the 
dynamical equilibrium. 

The writer some time ago suggested a modification 
of Secchi's theory, which seems to remove some of the 
objections, and appears on the whole more probable 
than any of the others. It may be that the spots are 
depressions in the photospheric level, caused not directly 
by the pressure of the erupted materials from above, 
but by the diminution of upward pressure ivova below, 
in consequence of eruptions in the neighborhood ; the 
spots thus being, so to speak, sinks in the photosphere. 
Undoubtedly the photosphere is not a strictly continu- 
ous shell or crust, but it is heavy as compared with the 



188 THE SUN. 

nncondensed vapors in which it lies, just as a rain-cloud 
in our terrestrial atmosphere is heavier than the air, 
and it is probably continuous enough to have its upper 
level affected by any diminution of pressure below. 
The gaseous mass below the photosphere supports its 
weight and the weight of the products of condensation, 
which must always be descending in an inconceivable 
rain and snow of molten and crystallized material. To 
all intents and purposes, though nothing but a layer of 
clouds, the photosphere thus forms a constricting shell, 
and the gases beneath are imprisoned and compressed. 
Moreover, at a high temperature the viscosity of gases 
is vastly increased, so that quite probably the matter of 
the solar nucleus resembles pitch or tar in its consist- 
ency more than what we usually think of as a gas. 
Consequently, any sudden diminution of pressure would 
propagate itself gradually from the point where it oc- 
curred. Putting these things together, it would seem 
that, whenever a free outlet is obtained through the 
photosphere at any point, thus decreasing the inward 
pressure, the result w^ould be the sinking of a portion 
of the photosphere somewhere in the immediate neigh- 
borhood, to restore the equilibrium ; and, if the erup- 
tion were kept up for any length of time, the depression 
in the photosphere would continue till the eruption 
ceased. Tliis depression, filled with the overlying gases, 
would constitute a spot. Moreover, the line of frac- 
ture, if we may call it so, at the edges of the sink would 
be a region of weakness in the photosphere, so that we 
should expect a series of eruptions all around the spot. 
For a time the disturbance, therefore, would grow, and 
the spot would enlarge and deepen, until, in spite of 
the viscosity of tlie internal gases, the equilibrium of 
pressure was gradually restored beneath. So far as we 



SUN-SPOT THEORIES. 189 

know the spectroscopic and visual phenomena, none of 
them contradict this hypothesis. 

As regards the limitation of the spots to certain 
latitudes, this, as has been said already, almost certainly 
will find its explanation in that of the equatorial accel- 
eration. Faye, Belopolsky, Lockyer, and Schaeberle, 
all present such explanations. Schaeberle's discussion 
of the subject may be found in " Astronomy and Astro- 
Physics," for April, 1894. We shall have occasion to 
refer to it again in connection with the corona. 

Whatever may be the cause of spots, it is probable 
that the annexed figure gives a fair idea of the arrange- 
ment and relations of the photospheric clouds in the 

Fig. 45. 




Constitution of a Sun- Spot. 



neighborhood of one. Over the sun's surface gener- 
ally, these clouds probably have the form of vertical 
columns, as at a a. Just outside the spot, the level of 
the photosphere is usually raised into faculse, as at J 5. 
These faculse are for the most part overtopped by 
eruptions of hydrogen and metallic vapors, as indicated 
by the shaded clouds. Of these metallic eruptions we 
shall have more to say in the chapter upon the chromo- 
sphere and prominences, only remarking here that, 



190 THE SUN. 

while the great clouds of hydrogen are found every- 
where upon the sun, these spiky, vivid outbursts of 
metallic vapors seldom occur, except just in the neigh- 
borhood of a spot, and then only during its season of 
rapid change. In the penumbra of the spot the photo- 
spheric filaments become more or less nearly horizontal, 
as ^^pp ; in the umbra, at u^ it is quite uncertain what 
the true state of affairs may be. We have con jecturally 
represented the filaments there as vertical also, but de- 
pressed and carried down by a descending current. Of 
course, the cavity o o\% filled by the gases which overlie 
the photosphere ; and it is easy to see that, looked at 
from above, such a cavity and arrangement of the 
luminous filaments would present the appearances actu- 
ally observed. 

Oppolzer, of Vienna, in 1893 proposed a new theory 
based largely upon Hann's researches on the tempera> 
ture effects of vertical atmospheric currents. Such cur-^ 
rents are supposed to rise periodically from the polar 
regions of the sun, to drift slowly toward its equator, 
and to descend in the spot zones, hecoming heated and 
" dried^'^ in their descent^ thus forming in the photo- 
sphere cavities which are filled with metallic vapors in 
purely gaseous condition. 

In many ways the theory admirably corresponds 
with facts, explaining better than any other the peculiar 
character of the sun-spot spectrum, Spoerer's law of 
sun-spot latitudes, and the otherwise puzzling observa- 
tions of Langley and Frost upon sun-spot temperatures 
(page 170, note). But the polar streams themselves are 
unaccounted for, and it remains to be seen how this 
'^ meteorological " theory will withstand other adverse 
criticisms. 



CHAPTEE VI. 

THE CHROMOSPHERE AND THE PROMINENCES. 

Early Observations of Chromosphere and Prominences. — The Eclipses of 
1842, 1851, and I860.— The Eclipse of 1868.— Discovery of Janssen 
and Lockyer. — Arrangement of Spectroscope for Observations upon 
Chromosphere. — Spectrum of Chromosphere. — Lines always present. 
—•Lines often reversed. — Ultra- Yiolet Studies of Hale and Des- 
landres. — Motion Forms. — Double Eeversal of Lines. — Distribution 
of Prominences. — Magnitude. — Classification of Prominences as qui- 
escent, and eruptive or metallic. — Isolated Clouds. — Violence of Mo- 
tion. — Observations of August 5, 1872. — Theories as to the Forma- 
tion and Causes of the Prominences. 

What we see of the sun under ordinary circum- 
stances is but a fraction of his total bulk. "While by 
far the greater portion of the solar mass is included 
within the photosphere — the blazing cloud-layer, which 
seems to form the sun's true surface, and is the princi- 
pal source of his light and heat — yet the larger portion 
of his volume lies without, and constitutes an atmos- 
phere whose diameter is at least double, and its bulk 
therefore sevenfold that of the central globe. 

Atmosphere, however, is hardly the proper term ; 
for this outer envelope, though gaseous in the main, is 
not spherical, but has an outline exceedingly irregular 
and variable. It seems to be made up not of overlying 
strata of different density, but rather of flamed, beams, 
and streamers, as transient and unstable as those of our 
own aurora borealis. It is divided into two portions, 
separated by a boundary as definite, though not so 



192 THE SUN, 

regular, as that which parts them both from the photo- 
sphere. The outer and far more extensive portion, 
which in texture and rarity seems to resemble the tails 
of comets, and may almost, without exaggeration, be 
likened to " the stuff that dreams are made of,'^ is 
known as the " coronal atmosphere," since to it is 
chiefly due the " corona " or glory which surrounds the 
darkened sun during an eclipse, and constitutes the 
most impressive feature of the occasion. 

At its base, and in contact with the photosphere, is t j 
what resembles a sheet of scarlet fire. The appearance, 
which probably indicates a fact, is as if countless jets 
of heated gas were issuing through vents and spiracles 
over the whole surface, thus clothing it with flame 
which heaves and tosses like the blaze of a conflagra- 
tion— "like a prairie on fire," to quote the vividly de- ^ 
scriptive phrase of Professor Langley. || 

This has received the name of chromosphere, a 
designation first proposed by Frankland and Lockyer 
in 1869, and signifying '^color-sphere," in allusion to 
the vivid redness of the stratum, caused by the pre- 
dominance of hydrogen in these flames and clouds. It || 
was called the " sierra ^^ by Airy in 1842, and Proctor 
and some other writers prefer tliat name to the later 
and more common appellation. 

Here and there masses of this hydrogen mixed with 
other substances rise to a great height, ascending far 
above the general level into the coronal regions, where 
they float like clouds, or are torn to pieces by contend- I 
ing currents. These cloud-masses are known as solar 
" prominences," or " protuberances," a non-committal 
sort of appellation applied in 1842, when they first 
attracted any considerable attention, and while it was 
a warmly-disputed question whether they were solar. 



II 



THE CHROMOSPHERE AND THE PROMINENCES. I93 

lunar, phenomena of our own atmosphere, or even mere 
optical illusions. It is unfortunate that no more appro- 
priate and graphic name has yet been found for objects 
of such wonderful beauty and interest. 

Until recently, the solar atmosphere could be seen 
only at an eclipse^ when the sun itself is hidden by the 
moon. Now, however, the spectroscope has brought 
the chromosphere and the prominences within the range 
of daily observation, so that they can be studied with 
nearly the same facility as the spots and faculae, and 
a fresh field of great interest and importance is thus 
opened to science. 

It seems hardly possible that the ancients should 
have failed to notice^ even with the naked eye, in some 
one of the many eclipses on record, the presence of 
blazing, star-like objects around the edge of the moon, 
but we find no mention of any thing of the kind, al- 
though the corona is described as we see it now. On 
this ground some have surmised that the sun has really 
undergone a change in modern times, and that the 
chromosphere and prominences are a new development 
in the solar history. But such mere negative evidence 
is altogether insufficient as a foundation for so impor- 
tant a conclusion. 

The earliest recorded observation of the prominences 
is probably that of Yassenius, a Swedish astronomer, 
who, during the total eclipse of 1733, noticed three or 
four small pinkish clouds, entirely detached from the 
limb of the moon, and, as he supposed, floating in the 
lunar atmosphere. At that time this was the most 
natural interpretation of the appearance, since the fact 
that the moon has no atmosphere was not yet ascer- 
tained. 

The Spanish admiral, Don Ulloa, in his account of 
14 



194 THE SUN. 



1 



the eclipse of 1778, describes a point of red light which 
made its appearance on the western limb of the moon 
about a minute and a quarter before the emergence of 
the sun. At first small and faint^ it grew brighter and 
brighter until extinguished by the returning sunlight. 
He supposed that the phenomenon was caused by a 
hole or fissure in the body of the moon ; but, with our 
present knowledge, there can be little doubt that it was 
simply a prominence gradually uncovered by her motion. 

The chromosphere seems to have been seen even 
earlier than the prominences : thus Captain Stannyan, 
in a report on the eclipse of 1706, observed by him at 
Berne, noticed that the emersion of the sun was pre- 
ceded by a blood-red streak of light, visible for six or 
seven seconds upon the western limb. Halley and 
Louville saw the same thing in 1715. Halley says that 
two or three seconds before the emersion a long and 
very narrow streak of a dusky but strong red light 
seemed to color the dark edge of the moon on the 
western edge where the sun was about to reappear. 
Louville's account agrees substantially with this, and 
he further describes the precautions he used to satisfy 
himself that the j^henomenon was no mere optical illu- 
sion, nor due to any imperfection of his telescope. 

In eclipses that followed that of 1733, the chromo- 
sphere and prominences seem to have attracted but lit- 
tle attention, even if they were observed at all. Some- 
thing of the sort appears to have been noticed by Ferrers 
in 1806, but the main interest of his observation lay in 
a different direction. 

In July, 1842, a great eclipse occurred, and the 
shadow of the moon described a wide belt running 
across southern France, northern Italy, and a portion 
of Austria. The eclipse was carefully observed by 



THE CHROMOSPHERE AND THE PROMINENCES. I95 

many of the most noted astronomers of the world, and 
so completely had previous observations of the kind 
been forgotten, that the prominences, which appeared 
then with great brilliance, were regarded with extreme 
surprise, and became objects of warm discussion, not 
only as to their cause and location, but even as to their 
very existence. Some thought them mountains upon 
the sun, some that they were solar flames, and others, 
clouds floating in the sun's atmosphere. Others re- 
ferred them to the moon, and yet others claimed that 
they were mere optical illusions. At the eclipse of 
1851 (in Sweden and Norway), similar observations 
were repeated, and, as a result of the discussions and 
comparison of observations which followed, astronomers 
generally became satisfied that the prominences are real 
phenomena of the solar atmosphere, in many respects 
analogous to our terrestrial clouds; and several came 
more or less confidently to the conclusion, now known 
to be true (see Grant's " History of Physical Astrono- 
my "), that the sun is entirely surrounded with a con- 
tinuous stratum of the same substance. Many, how- 
ever, remained unconvinced : Faye, for instance, still 
asserted them to be mere optical illusions, or mirages. 

In the eclipse of 1860, photography was for the first 
time employed on such an occasion with anything like 
success. The results of Secchi and De La Eue removed 
all remaining doubts as to the real existence and solar 
character of the objects in question, by exhibiting them 
upon their plates gradually covered on one side and un- 
covered on the other side of the sun by the progress of 
the moon. 

Secchi thus sums up his conclusions, which have 
been justified in almost all their details by later obser- 
vations ; they require few and slight corrections : 



196 THE SUN. 

" 1. The prominences are not mere optical illusions ; 
the J are real phenomena pertaining to the sun. . . . 

2. The prominences are collections of luminous mat- 
ter of great brilliance, and possessing remarkable pho- 
tographic activity. This activity is so great that many 
of them, which are visible in our photographs, could 
not be seen directly even with good instruments. 

3. Some protuberances float entirely free in the so- 
lar atmosphere like clouds. If they are variable in form, 
their changes are so gradual as to be insensible in the 
space of ten minutes. (Generally, but by no means al- 
ways, true.) 

4. Besides the isolated and conspicuous protuber- 
ances there is also a layer of the same luminous sub- 
stance which surrounds the whole sun, and out of which 
the protuberances rise above the general level of the so- 
lar surface. . . . 

5. The number of the protuberances is indefinitely 
great. In direct observation through the telescope the 
sun appeared surrounded with flames too numerous to 
count. . . . 

6. The height of the protuberances is very great, 
especially when we take account of the portion hidden 
by the moon. One of them had a height of at least 
three minutes, which indicates a real altitude of more 
than ten times the earth's diameter. ..." 

But their nature still remained a mystery ; and no 
one could well be blamed for thinking it must always 
remain so to some degree. At that time it could hard- 
ly be hoped that we should ever be able to ascertain 
their chemical constitution, and measure the velocities 
of their motions. And yet this has been done. Before 
the great Indian eclipse of August 18, 1868, the spec- 
troscope had been invented (it was, indeed, already in 



THE CHROMOSPHERE AND THE PROMINENCES. 197 

its infancy in I860), and applied to astronomical researcli 
with the most astonishing and important results. 

Every one is more or less familiar with the story of 
this eclipse. Herschel, Tenriant, Pogson, Eayet, and 
Janssen, all made substantially the same report. They 
found the spectrum of the prominences observed to con- 
sist of bright lines, and conspicuous among them were 
the lines of hydrogen. There were some serious dis- 
crepancies, indeed, among their observations, not only 
as to the number of the bright lines seen, which is not 
to be wondered at, but as to their position. Thus, 
Rayet (who saw more lines than any one else) identified 
the red line observed with B instead of C ; and all the 
observers mistook the yellow line they saw for that of 
sodium. 

Still, their observations, taken together, completely 
demonstrated the fact that the prominences are enor- 
mous masses of highly-heated gaseous matter, and that 
hydrogen is a main constituent. 

Janssen went further. The lines he saw during the 
eclipse were so brilliant that he felt sure he could see 
them again in the full sunlight. He was prevented by 
clouds from trying the experiment the same afternoon, 
after the close of the eclipse ; but the next morning the 
sun rose unobscured, and, as soon as he had completed 
the necessary adjustments, and directed his instrument 
to the portion of the sun's limb where the day before 
the most brilliant prominence appeared, the same lines 
came out again, clear and bright ; and now, of course, 
there was no difficulty in determining at leisure, and 
with almost absolute accuracy, their position in the 
spectrum. He immediately confirmed his first conclu- 
sion, that hydrogen is the most conspicuous component 
of the prominences, but found that the yellow line must 



198 THE SUN. 



1 



be referred to some other element than sodium, being 
somewhat more refrangible than the D lines. 

He found also that, by slightly moving his telescope 
and causing the image of -the sun's limb to take different 
positions with reference to the slit of his spectroscope, 
he could even trace out the form and measure the 
dimensions of the prominences ; and he remained at 
his station for several days, engaged in these novel and 
exceedingly interesting observations. 

Of course, he immediately sent home a report of his 
eclipse- work, and of his new discovery, but, as his sta- 
tion at Guntoor, in eastern India, was farther from 
mail communication with Europe than those upon the 
western coast of the peninsula, his letter did not reach 
France until some week or two after the accounts of 
the other observers; when it did arrive, it came to 
Paris, in company w^ith a communication from Mr. 
Lockyer, announcing the same discovery, made inde- 
pendently, and even more creditably, since with Mr. 
Lockyer it was not suggested by anything he had seen, 
but was thought out from fundamental principles. 

Nearly two years previously the idea had occurred 
to him (and, indeed, to others also, though he was the 
first to publish it) that, if the protuberances are gaseous, 
so as to give a spectrum of bright lines, those lines 
ought to be visible in a spectroscope of sufficient power, 
even in broad daylight. The principle is simply this : 

Under ordinary circumstances the protuberances are 
invisible, for the same reason *as the stars in the day- 
time : they are hidden by the intense light reflected 
from the particles of our own atmosphere near the 
sun's place in the sky, and, if we could only sufficiently 
weaken this aerial illumination, without at the same 
time weakening their light, the end would be gained. 



THE CHROMOSPHERE AND THE PROMINENCES. 199 

And the spectroscope accomplishes precisely this very 
thing. Since the air-light is reflected sunshine, it of 
course presents the same spectrum as sunlight, a con- 
tinuous band of color crossed by dark lines. I^ow, this 
sort of spectrum is greatly weakened by every increase 
of dispersive power, because the light is spread out into 
a longer ribbon and made to cover a more extended 
area. On the other hand, a spectrum of bright lines 
undergoes no such weakening by an increase in the dis- 
persive power of the spectroscope. The bright lines 
are only more widely separated — not in the least dif- 
fused or shorn of their brightness. Moreover, if the 
gas is one which, like hydrogen, shows dark lines in 
the ordinary solar spectrum (and therefore in that of 
the air-light), the case is even better : not only is* the 
continuous spectrum of the air-light w^eakened by the 
high dispersion, but it has dark gaps in it just wliere 
the bright lines of the prominence spectrum will fall. 

If, then, the image of the sun, formed by a telescope, 
be examined with a spectroscope, one might hope to 
see at the edge of the disk the bright lines belonging 
to the spectrum of the prominences, in case they are 
really gaseous. 

Mr. Lockyer and Mr. Huggins both tried the experi- 
ment as early as 1867, but without success ; partly be- 
cause their instruments had not sufiicient power to bring 
out the lines conspicuously, but more because they did 
not know whereabouts in the spectrum to look for them, 
and were not even sure of their existence. At any rate, 
as soon as the discovery was announced, Mr. Huggins 
immediately saw the lines without difficulty, with the 
same instrument which had failed to show them to him 
before. It is a fact, too often forgotten, that to per- 
ceive a thing known to exist does not require one half 



200 THE SUN. 

the instrumental power or acuteness of sense as to dis- 
cover it. 

Mr. Lockyer, immediately after his suggestion was 
published, had set about procuring a suitable instrument, 
and was assisted by a grant from the treasury of the 
Eoyal Society. After a long delay, consequent in part 
upon the death of the optician who had first under- 
taken its construction, and partly due to other causes, 
he received the new spectroscope just as the report of 
Herschel's and Tennant's observations reached England. 
Hastily adjusting the instrument, not yet entirely com- 
pleted, he at once applied it to his telescope, and with- 
out difficulty found the lines, and verified their position. 
He immediately also discovered them to be visible 
around the whole circumference of the sun, and conse- 
quently that the protuberances are mere extensions of 
a continuous solar envelope, to w^hich, as mentioned 
above, was given the name of Chromosphere. (He does 
not seem to have been aware of the earlier and similar 
conclusions of Arago, Grant, Secchi, and others.) He 
at once communicated his results to the Royal Society, 
and also to the French Academy of Sciences, and, by 
one of the curious coincidences which so frequently 
occur, his letter and Janssen's were read at the same 
meeting, and within a few minutes of each other. 

The discovery excited the greatest enthusiasm, and 
in 1872 the French Government struck a gold medal 
in honor of the two astronomers, bearing their united 
effigies. 

It immediately occurred to several observers, Jans- 
sen, Lockyer, Zollner, and others, that by giving a rapid 
motion of vibration or rotation to the slit of the spec- 
troscope it would be possible to perceive the whole con- 
tour and detail of a protuberance at once, but it seems 



II 



THE CHROMOSPHERE AND THE PROMINENCES. 201 

to have been reserved for Mr. Huggins to be the first 
to show practically that a still simpler device would 
answer the same purpose. With a spectroscope of suf- 
ficient dispersive power it is only necessary to widen 
the slit of the instrument by the proper adjusting screw. 
As the slit is widened, more and more of the protuber- 
ance becomes visible, and, if not too large, the whole 
can be seen at once : with the widening of the slit, how- 
ever, the brightness of the background increases, so 
that the finer details of the object are less clearly seen, 
and a limit is soon reached beyond w^hich further widen- 
ing is disadvantageous. The higher the dispersive pow- 
er of the spectroscope the wider the slit that can be 
used, and the larger the protuberance that can be exam- 
ined as a whole — within certain limits, however. It is 
not difficult with our latest spectroscopes, diifraction 
instruments especially, to reach a dispersion so great 
that even the C line becomes broad and hazy, like the 

Fig. 46. 




Htjggtns's FiEST Obsrrvation of a Prominence in Full Sunshine. 

h lines in an ordinary instrument. In that case each 
luminous point in the prominence itself is represented 
in the image of the prominence, net by a point, as it 
should be to give clear definition, but by a streak at 
right angles to the spectrum lines. 

Mr. Huggins's first successful observation of the 



202 



THE SUN. 



form of a solar protuberance was made on February 13, 
1869. Fig. 46, copied from the "Proceedings of the 
Eoyal Society," presents his delineation of what he saw. 
As his instrument liad only the dispersive power of two 
prisms, and included in its field of view a large portion 
of the spectrum at once, he found it necessary to sup- 
plement its powers by using a red glass to cut off stray 



Fig. 47. 




Spectroscope, with Train of Pkisms. 



11 atl 



light of other colors, and by inserting a diaphragm 
the focus of the small telescope of the spectroscope to 
limit the field of view to the portion of the spectrum 
immediately adjoining the C line. With the instru- 
ments now in use, these precautions are seldom neces-| 
sary. 

It may be noticed, in passing, that Mr. Huggins had . ^ 
previously (and has subsequently) made many experi-l 
ments with different absorbing media, in hopes of find- 
ing some substance which, by cutting off all light of 
other color than that emitted by the prominences, should 
render them visible in the telescope ; thus far, however, 
without success. 

The spectroscopes used by difl'erent astronomers for 



THE CHROMOSPHERE AND THE PROMINENCES. 203 

observations of tliis sort differ greatly in form and 
power. Fig. 47 represents the one long used at the 
Shattuck Observatory of Dartmouth College, and sev- 
eral of our American observatories are supplied with 
instruments similarly arranged. The light passes from 
the collimator c^ through the train of prisms p^ near 
their bases, and, by two reflections in a rectangular 
prism, ^, is transferred to the upper story, so to speak, 
of the prism-train, and made to return to the telescope 
^, finally reaching the eye at e. It thus twice traverses 
a train of six prisms, and the dispersive power of the 
instrument is twelve times as great as it would be with 
only one prism. The diameter of the collimator is a 
i little less than an inch, and its length ten inches. The 
[ whole instrument only weighs about fourteen pounds, 
and occupies a space of about 15 in. X 6 in. X 5 in. 
It is also automatic^ i. e., the tangent screw m keeps 
I tlie train of prisms adjusted to their position of min- 
i imum deviation by the same movement which brings 
I the different portions of the spectrum to the center of 
the field of view, and the milled head f focuses both 
I the collimator and telescope simultaneously. 

The spectroscope is attached to the equatorial tele- 
i scope, to which it belongs, by means of the clamping 
j rings a^ a. These slide upon a stout metal rod, firmly 
I fastened to the telescope in such a way that the slit s^ 
of the instrument, can be placed exactly at the focus of 
j the object-glass, w^here the image of the sun is formed. 
\ This instrument, attached to the telescope, has already 

ibeen figured upon page 74. 
Instruments in which the prism-train is replaced by 
, a diffraction-grating are still more powerful ; and more 
convenient also, since the observer has the great advan- 
tage of being able to select, within certain limits, the 



204 THE SUN. 

amount of dispersion best suited to his purpose by sim- 
ply turning the grating so as to utilize the different 
orders of spectra — an operation easier and more rapid 
than that of rearranging the prism-train. Diffraction 
spectroscopes have, however, one slight disadvantage. 
When used with the open slit, the forms of objects seen 
through the slit are somewhat distorted, being either 
compressed or extended in a direction at right angles 
to the slit. When the grating is so placed that the 
inclination of its surface to the view-telescope is greater 
than to the collimator (as in the figure on page 68), 
compression occurs. In this case, the edge of the slit 
being placed tangential to the sun's limb, as is usual, 
prominences on the edge of the sun appear to have their 
height reduced. Of course, the reverse takes place when 
the grating is placed the opposite way. This distortion, 
however, is of little importance, as its amount is easily 
calculated and allowed for when necessary.* A similar 
distortion is produced by prismatic spectroscopes when 
the prisms are not adjusted strictly to their position 
of minimum deviation. 

The diffraction instruments, which the writer is ac- 
customed to use for solar observations at Princeton, 
have already been figured on pages 69 and 76. 

With a telescope of not less than four inches aper- 
ture, equatorially mounted, and a spectroscope of dis-j 
persive power not less than that of five or six ordinary 
prisms, the observer is equipped for the study of the 
chromosphere and prominences. He may either study 
the spectrum as such, using the instrument with a nar- 

* The formula for the calculation is simply H = 7i . ' , •> in which H 

sm. K 

is the true height of the object seen throujrh the slit; h is its apparent 

height, and k and t are the inclinations of the surface of the grating to 

the collimator and view-telescope respectively. 



1 



THE CHROMOSPHERE AND THE PROMINENCES. 205 

row slit, or he may employ it with widened slit simply 
as a means of viewing the prominences and studying 
their forms and changes, and, so far as this goes — the 
study of forms and changes — very little is gained by 
large telescopes and high dispersion. The writer has 
never had finer views of prominences than those fur- 
nished by the old Dartmouth telespectroscope. When 
it comes, however, to the study of minute details, espe- 
cially details of the spectrum, then large instruments 
are indispensable. 

SPECTRUM OF THE CHROMOSPHERE AND PROMINENCES. 

The spectra of the chromosphere and prominences 
are very interesting in their relations to that of the 
photosphere, and present many peculiarities which are 
not yet fully explained. At times and in places where 
some special disturbance is going on — frequently in the 
neighborhood of spots at the times when they are just 
passing around the limb of the disk — the spectrum, at 
the base of the chromosphere, is very complicated, con- 
sisting of hundi-eds of bright lines. In the course of a 
few weeks of observation at Sherman in 1872, the writer 
made out a list of two hundred and seventy-three, and 
more recent observations have added largely to the 
number — at least fifty lines within the limits of the 
visible spectrum, and, by photography, at least eighty 
in the ultra-violet. The majority of the lines, how- 
ever, are seen only occasionally, for a few minutes 
at a time, when the gases and vapors, which generally 
lie low, mainly in the interstices of the clouds which 
constitute the photosphere, and below its upper surface, 
are elevated for the time being by some eruptive action. 
For the most part, the lines which appear only at such 
times are simply "reversals" of the more prominent 



206 THE SUN. 

dark lines of the ordinary solar spectrum. But the se- 
lection of the lines seems most capricious ; one is taken, 
and another left, though belonging to the same element, 
of equal intensity, and close beside the first. It is evi- 
dent that the subject needs a detailed and careful study, 
combining solar observations with laboratory- work upon 
the spectra of the elements concerned, before a satis- 
factory account can be given of all the peculiar behav- 
ior observed. 

The lines composing the true chromosphere spec- 
trum, if we may call it so (that is, those which are 
always observable in it with suitable appliances), are 
not very namerous, and we give the following list, 
designating them by their wave-length, as given by 
Rowland : 

1. 7065-50. Helium. 

2. 6563-05, C. Hydrogen (Ha). 

3. 5875-98, D3. (close double). Helium. 

4. 5316-87. The corona-line. "Coronium." ? 

5. 4861-50, F. Hydrogen (HjS). 

6. 4471-80, /. Helium. 

7. 4840-66, ff (near G). Hydrogen (H7). 

8. 4101-85, h. - Hydrogen (H5). 

9. 3970-20 (in H). Hydrogen (He). 

10. 3968-56, H. Calcium. 

11. 3933-86, K. Calcium. 

The first line is generally very diSicult to see, though 
sometimes pretty conspicuous. It is in the red, between 
B and a^ and has a very faint corresponding dark line. 
No. 3 has no dark line corresponding as a usual thing, 
though occasionally one appears, especially in the neigh- 
borhood of sun-spots. No. 9 is quite within the broad 
shade of the H-line, which thus appears double in the 
chromosphere spectrum. 

The eleven lines mentioned above are invariably 



THE CHROMOSPHERE AND THE PROMINENCES. 207 



present in the spectrum of tlie cliromospliere ; a much 
larger number make their appearance on very shght 
provocation. They are : 



r. 


6678-2. 


Helium. 


18' 


4924-1. 


Iron. 


2'. 


6431-1. 


Iron. 


19' 


4922-3. 


Helium. 


3'. 


6141-9. 


Barium. 


20' 


4919-1. 


Iron. ? 


4. 


5896-2, Di. 


Sodium. 


21' 


4900-3. 


Barium. 


5'. 


5890-2, D2. 


Sodium. 


22' 


4584-1. 


Iron. 


G'. 


5363-0. 


Iron. ? 


23'. 


4501-4. 


Titanium. 


r. 


5284-6. 


Titanium ? ? 


24' 


4491-5. 


Manganese. 


8'. 


5276.2. 


Chromium. ? 


25'. 


4490-2. 


Manganese. 


9'. 


5234-7. 


Manganese. 


26'. 


4469-5. 


Iron. 


10'. 


5198-2. 


? ? 


27'. 


4245-5. 


Iron. 


11'. 


5183-8, h,. 


Magnesium. 


28'. 


4236-1. 


Iron. 


12'. 


5172-9, h^. 


Magnesium. 


29'. 


4233-8. 


Iron. 


13'. 


5169-2, 63. 


Iron. 


80'. 


4226-9. 


Calcium. 


14'. 


5167-6, 64. 


Magnesium. 


31'. 


4215-7. 


Strontium. 


15'. 


5018-6. 


Iron. 


32'. 


4077-9. 


Calcium. 


16'. 


5015-8. 


Helium. 


83'. 


4026-0. 


Helium. 


17'. 


4934-3. 


Barium. 


34'. 


3889-1. 


Hydrogen (HQ 



It is not intended, however, to intimate that, if one 
of these appears, all of them will do so, nor that they 
are equally conspicuous or equally common. To a cer- 
tain degree, also, their selection by the writer is arbi- 
trary, for there are nearly as many more which are seen 
pretty frequently, and some of them may very possibly 
be found hereafter to deserve a place upon the list 
rather than some that have been included. 

It requires careful manipulation to bring out the 
fainter and finer lines satisfactorily. The slit must be 
adjusted wdth extreme care to the focal plane of the 
rays under examination, placed tangential to the solar 
image, and brought exactly to the edge of the disk. A 
thousanth of an inch in its position w^ll often make the 
whole difference between a successful operation and its 
failure, and even a slight unsteadiness of the air w^ill 



208 



THE SUN. 



diminish the number of bright lines visible by at least 
one half. 

As the majority of the lines are developed only by 
more or less unusual disturbances of the solar surface, 
it naturally happens that one very often finds them dis- 
torted or displaced by the motions of the gases along] 
the line of sight (toward or from the observer), as ex- 
plained in a previous chapter, producing what Lockyerj 
calls " motion-forms." Occasionally, also, we meet with 
"double reversals," so called, especially in the lines of 
magnesium and sodium. The (dark) lines of these sub- 
stances are rather wide in the solar spectrum. When 
reversed in the chromosphere spectrum, the phenome- 
non usually consists of a thin bright line down the cen- 
ter of the wider dark band : in a double reversal the 
bright line widens and a fine dark line appears in its 
center, so that we have a central dark line, a bright one 
on each side of it, and outside of the bright lines a dark 
shade on both sides. Fig. 48 represents such a double 

Fig. 4S. 



Double-reversal of the D-Lines.— (October. 1880.) 

reversal of the D-lines observed by the writer on several 
occasions in 1880. The phenomenon seems to be due 
to the presence of an unusual quantity of the vapor at a 
considerable density, and is the precise correlative of 



THE CHROMOSPHERE AND THE PROMINENCES. 209 

what is sometimes seen in the spectrum of a sodium- 
flame. The two D-lines of sodium each becomes itself 
double, so that we get pairs of bright lines in place of 
single lines. The electric arc often shows this still 
more finely. 

At the base of a prominence, the C, F, H, and K 
lines are always thus doubly reversed. Fig. 49 is from 

Fig. 49. 




DOFBLE-REVBKSAL OF C-LlNB.— (PhOTOGJRAPHED.) 

a recent photograph of the C-line obtained at Prince- 
ton, by Mr. Reed, with the large telescope and spectro- 
scope. The slit was tangential to the sun's limb. Of 
course, an isochromatic plate and a long exposure were 
required to get such an impression from the "ruby 
light " of that part of the spectrum. When the slit is 
adjusted to cross the sun's limb radially the bright 
lines where they project beyond the spectrum of the 
photosphere assume the "arrow-headed" form shown 
in Fig. 50. 

Generally speaking, the spectrum of a prominence 
15" 



210 



THE SUN. 



is simpler than that of the chromosphere at its base. 
We seldom find any lines except C, Dg, F, g^ A, 11 and 



Fig. 50. 




K, at a considerable elevation above the photosphere, 
though f is sometimes met with. On rare occasions, 
also, the vapors of sodium and magnesium are carried 
into the higher regions, and once or twice the writer 
has seen the line No. 1 of the second list (6678'2) in the 
upper portions of a prominence. 



OBSERVATION OF PROMINENCES. 

When the spectroscope is used as a means of ren- 
dering visible the forms and features of the promi- 
nences, the only difference is that the slit is more or less 
widened. 

Fig. 51. 




Opened Slit of the Spectroscope. 



Tlie telescope is directed so that the solar image 
shall fall with that portion of its limb which is to be 



THE CHROMOSPHERE AND THE PROMINENCES. 211 

examined just tangent to the opened slit, as in Fig. 51, 
which represents the slit-plate of the spectroscope, with 
the image of the sun in position for observation. 

If, now, a prominence exists at this part of the sun's 
limb (as would probably be the case, considering the 
proximity of the spot shown in the figure), and if the 
spectroscope itself is so adjusted that the C-line falls in 
the center of the field of view, then, on looking into 
the eye-piece, one will see something much like Fig. 52. 
The red portion of the spectrum will stretch athwart 
the field of view like a scarlet ribbon, with a darkish 
band across it, and in that band will appear the promi- 
nences, like scarlet clouds — so like our own terrestrial 
clouds, indeed, in form and texture, that the resem- 
blance is quite startling : one might almost think he 
was looking out through a partly-opened door upon a 
sunset sky, except that there is no variety or contrast of 
color ; all the cloudlets are of the same pure scarlet hue. 
Along the edge of the opening is seen the chromo- 
sphere, more brilliant than the clouds which rise from 
it or float above it, and for the most part made up of 
minute tongues and filaments. Usually, however, the 
definition of the chromosphere is less distinct than that 
of the higher clouds. The reason is, that close to the 
limb of the sun, where, the temperature and pressure 
are highest, the hydrogen is in such a state that the 
lines of its spectrum are widened and " winged," some- 
thing like those of magnesium, though to a less extent. 
Each point in the chromosphere, therefore, when viewed 
through the opened slit, appears not as a pointy but as 
a short line, directed lengthwise in the spectrum. As 
the length of this line depends upon the dispersive 
power of the spectroscope, it is easy to see that it is 
possible to go too far in this respect. The lower the 



212 



THE SUN. 



1 



dispersion the more distinct the image obtained, but 
also the fainter as compared with the background upon 
which it is seen. 

Just beneath the chromosphere (at a in the cut) the 
appearance is as if the edge of the sun was dark^ a phe- 
nomenon which for some time was very puzzUng. Its 
explanation lies in the " double-reversal " of the C-line 
at the base of the chromosphere, discussed and figured a 
few pages back. 



Fig. 52. 




Chromosphere and Prominbnobb as seen in the Spectkitm. 

If the spectroscope is adjusted upon the F-line, in- 
stead of C, then a similar image of the prominences and 
chromosphere is seen, only blue instead of scarlet ; usu- 
ally, however, since the F-line is hazier and more winged 
than C, this blue image is somewhat less perfect in its 
details and definition, and is therefore less used for 
observation. Similar effects are obtained by means of 
the yellow line near D, and the violet line near G. 
With suitable precautions, using a violet shade-glass be- 
fore the eye, and carefully shutting out all extraneous 



THE CHROMOSPHERE AND THE PROMINENCES. 213 

light, the H and K lines can also be used ; but visual 
observations in this part of the spectrum are extremely 
difficult and unsatisfactory. 

With photography the case is the reverse — these 
lines are then precisely those which can be employed 
most easily and conveniently. We shall recur to this a 
little later. 

Professor Winlock and Mr. Lockyer have attempted, 
by using an annular opening instead of the ordinary 
slit, to obtain a view of the whole circumference of the 
sun at once, and have succeeded, Witli a spectroscope 
of sufficient power, and adjustments delicate enough, 
the thing can be done ; but as yet no very satisfactory 
results appear to have been reached. We still (in visual 
observations) have to examine the circumference piece- 
meal, so to speak, readjusting the instrument at each 
point, to make the slit tangential to the limb. 

The number of protuberances of considerable mag- 
nitude (exceeding ten thousand miles in altitude), visible 
at any one time on the circumference of the sun, is 
never very great, rarely reaching twenty-five or thirty. 
Their number, however, varies extremely with the num- 
ber of sun-spots : during a sun-spot minimum there are 
not unfrequently occasions when not a single one can 
be found, though even during those years the more 
usual number is five or six — some of which often are 
of considerable size. The observations of Tacchini and 
Secchi have showed that their numbers closely follow 
the march of the sun-spots, though never falHng quite 
so low. 

To Tacchini we owe our most complete record of 
these objects, now continuous since 1872, giving their 
number and distribution upon the sun, with drawings 
of all that were specially remarkable. Many others 



214 



THE SUN. 



have co-operated in observations of this kind : the Hun- 
garian observers, Fenyi at Kalocsa, and Yon Gothard at 
Hereny, have given us many fine descriptions and de- 
lineations. Father Perry and his assistant Sidgreaves, 
at Stonyhurst, also deserve a special mention. 

Their distribution on the sun's surface is in some 
respects similar to that of the spots, but with important 
differences. The spots are confined within 40° of the 
sun's equator, being most numerous at a solar latitude 
of about 20° on each hemisphere. Now, the protuber- 



FiG. 53. 




Relative Frequency of Protuberances and Sun-Spots. 

ances are most numerous precisely where the spots are 
most abundant, but they do not disappear at a latitude 
of 40° ; they are found even at the poles, and from the 
latitude of 60° actually increase in number to a latitude 
of about 75°. 



4 



THE CHROMOSPHERE AND THE PROMINENCES. 215 

The annexed diagram, Fig. 53, represents the rela^ 
tive frequency of the protuberances and spots on the 
different portions of the solar surface. On tlie left side 
is given the result of Carrington's observation of 1,386 
spots between 1853 and 1861, and on the right the re- 
sult of Secchi's observations of 2,767"^ protuberances in 
1871. The length of each radial line represents the 
number of spots or protuberances observed at each par- 
ticular latitude on a scale of a quarter of an inch to thfe 
hundred ; for example, Secchi gives 228 protuberances 
as the number observed during the period of his work 
between 10° and 20° of south latitude, and the corre- 
sponding line drawn at 15° south, on the left-hand side 
of the figure, is therefore made |f f or '57 of an inch 
long. The other lines are laid off in the same way, and 
thus the irregular curve drawn through their extremities 
represents to the eye the relative frequency of these 
phenomena in the different solar latitudes. The dotted 
line on the right-hand side represents in the same man- 
mer and on the same scale the distribution of the larger 
protuberances, having an altitude of more than 1\ or 
27,000 miles. 

A mere inspection of the diagram shows at once 
that, while the prominences may, and in fact often do, 
have a close connection with the spots, they are yet to 
some extent independent phenomena. 

A careful study of the subject shows that they are 
much more closely related to the faculae.f In many 

* The 2,767 prominences are not all different ones. If any of the 
prominences observed on one day remained visible the next, they were 
recorded afresh ; and, as a prominence near the pole would be carried 
but slowly out of sight by the sun's rotation, it is thus easy to see how 
the number of prominences recorded in the polai^egions is so large. 

f See page 109. 



216 THE SUN. 

cases at least, faculae, when followed to the limb of the 
sun, have been found to be surrounded by prominences, 
and there is reason to suppose that the fact is a general 
one. The spots, on tlie other hand, when they reach 
the border of the sun's image, are commonly surround- 
ed by prominences more or less completely, but seldom 
overlaid by them. Indeed, Respiglii asserts (and the 
most careful observations w^e have been able to make 
confirm his statement) that as a general rule the chro- 
mosphere is considerably depressed immediately over a 
spot. Secchi, however, denies this. 

MAGNITUDE AND CLASSIFICATION OF PKOMINENCES. 

Tlie protuberances differ greatly in magnitude. The 
average depth of the chromosphere is not far from 10^' 
or 12^', or about 5,000 or 6,000 miles, and it is not, there- 
fore, customary to note as a prominence any cloud with 
an elevation of less than W or 20''— Y,000 to 9,000 
miles. Of the 2,Y67 already quoted, 1,964 attained an 
altitude of 40'', or 18,000 miles, and it is worthy of 
notice that the smaller ones are so few, only about one 
third of the whole : 751, or nearly one fourth of the 
whole, reached a height of over 1', or 28,000 miles ; the 
precise number which reached greater elevations is not 
mentioned, but several exceeded 3', or 84,000 miles. It 
is only rather rarely that they reach elevations as great 
as 100,000 miles. The writer has in all seen, perhaps, 
three or four which exceeded 150,000 miles, and Secchi 
has recorded one of 300,000 miles. On October Y, 1880, 
the writer observed one which attained the still un- 
equaled height of over 13' of arc, or 350,000 miles. 
AVhen first seen, on the soutlieast limb of the sun, about 
10.30 A. M., it was a " horn " of ordinary appearance, 
some 40,000 miles in elevation, and attracted no special 



THE CHROMOSPHERE AND THE PROMINENCES. 217 

attention. When next seen, half an hour later, it had 
become very brilliant and had doubled its height : dur- 
ing the next hour it stretched upward until it reached 
the enormous altitude mentioned, breaking up into fila- 
ments which gradually faded away, until, by 12.30 p. m.. 



EEUPTIYE PEOMINENCES. 



Three figures, of the same prominence, 

seen July 25. 1872. 

Fig. 54. 



Fig. 5T. 




As SEEN AT 3.30 P. M. 

100,000 miles to the inch, 



JbT8» 



218 THE SUN. 

there was notliing left. A telescopic examination of the 
sun's disk showed nothing to account for such an ex- 
traordinary outburst, except some small and not very 
brilliant faculse. While it was extending upward most 
rapidly a violent cyclonic motion was shown by the dis- 
placement of the spectrum-lines, and H and K were re- 
versed through its whole height. 

In their form and structure the protuberances differ 
as widely as in their magnitude. Two principal classes 
are recognized by all observers — the quiescent^ cloud- 
formed^ or hydrogenous, and the eruptive or metallic. 
By Secchi these are each further subdivided into several 
sub-classes or varieties, between which, however, it is 
not always easy to maintain the distinctions. 

And here perhaps is the proper place to mention 
that Trouvelot insists on the existence of ''dark" prom- 
inences — i. e., clouds of cooler hydrogen that absorb the 
light of the hydrogen behind them ; but there is no 
proof, we think, that these are anything but ''holes.'' 
Tacchini, on the other hand, is disposed to assert the 
existence of "white" prominences, which give a con- 
tinuous spectrum, and so are not reached by spectro- 
scopic observation, though conspicuous to the eye, and 
on the photographic plate, at the time of a total eclipse, 
as in 1883 and December, 1889. But the evidence hardly 
warrants confident belief in the existence of such ob- 
jects. 

The quiescent prominences in form and texture re- 
semble, with almost perfect exactness, our terrestrial 
clouds, and differ among themselves as much and in the 
same manner. The familiar cirrus and stratus types are 
very common, the former especially, while the cumulus 
and cumulo-stratus are less frequent. The protuber- 
ances of this class are often of enormous magnitude, 



THE CHROMOSPHERE AND THE PROMINENCES. 219 



QUIESCENT PROMINENCES. 

Scale, 75,000 miles to the inch. 

Ftg. 63. 



Fig. 


60. 




■^^-'-^" 


"t/'i^l^^y 


^'■w- ..,, 




^ •:#- r 


.^.^^ 


^.':.. .^ 


^^■HQBbk^k^:^^^' 


^«sn^^^^,^^ 


'-'f^^ ^^^^*^- 




Clouds. 



DlTFCSE. 



Ftg. 64. 



Fig. 61. 





Filamentary. 



Fig. 65. 




Plumes. 



Horns. 



220 THE SUN. 

especially in tlieir liorizontal extent (but the highest 
elevations are attained by those of the eruptive order), 
and are comparatively permanent, remaining often for 
hours and days without serious change ; near the poles 
they sometimes i:>ersist through a v^hole solar revolution 
of twenty-seven days. Sometimes they appear to lie 
upon the limb of the sun like a bank of clouds in the 
horizon ; probably because they are so far from the 
edge of the disk that only their upper portions are in 
sight. When seen in their full extent they are ordi- 
narily connected to the underlying chromosphere by 
slender columns, which are usually smallest at the base, 
and appear often to be made up of separate filaments 
closely intertwined, and expanding upward. Some-j 
times the whole under surface is fringed with down- 
hanging filaments, which remind one of a summer! 
shower falling from a heavy thunder-cloud. Some-] 
times they float entirely free from the chromosphere;" 
indeed, as a general rule, the layer clouds are attended 
by detached cloudlets for the most part horizontal in 
their arrangement. 

The figures give an idea of some of the general ap- 
pearances of this class of prominences, but their delicate, 
filmy beauty can be adequately rendered only by a far 
more elaborate style of engraving. 

Their spectrum is usually very simple, consisting of 
the four lines of hydrogen, and the three of helium, 
with H and K. Occasionally the sodium and mag- 
nesium lines also appear, and that even near the sum- 
mit of the clouds ; and this phenomenon was so much 
more frequently observed in the clear atmosphere of 
Sherman as to suggest that, if the power of our spec- 
troscopes were sufticiently increased, it would cease to 
be unusual. 



THE CHROMOSPHERE AND THE PROMINENCES. 221 

The genesis of this sort of prominence is problemat- 
ical. They have been commonly looked upon as the 
debris and relics of eruptions, consisting of gases which 
have been ejected from beneath the solar surface, and 
then abandoned to the action of the currents of the 
sun's upper atmosphere. But near the poles of the sun 
distinctively eruptive prominences never appear, and 
there is no evidence of aerial currents which would 
transport to those regions matters ejected nearer the 
sun's equator. Indeed, the whole appearance of these 
objects indicates that they originate where we see them. 
Possibly, although in the polar regions there are no 
violent eruptions, there yet may be a quiet outpouring 
of heated hydrogen suflBcient to account for their pro- 
duction — an outrush issuing through the smaller pores 
of the solar surface, which abound near the poles as 
well as elsewhere. 

But Secchi reports an observation which, if correct, 
puts a very different face upon the matter.* He has 
seen isolated cloudlets form and grow spontaneously 
without any perceptible connection with the chromo- 
sphere or other masses of hydrogen, just as in our own 
atmosphere clouds form from aqueous vapor, already 

* On October 13, 1880, the writer for the first time met with the 
same phenomenon. A small, bright cloud appeared on that day, about 
11 A.M., at an elevation of some 2|' (6Y,600 miles) above the limb, with- 
out any evident cause or any visible connection with the chromosphere 
below. It grew rapidly without any sensible rising or falling, and in an 
hour developed into a large stratiform cloud, irregular on the upper sur- 
face, but nearly flat beneath. From this lower surface pendent filaments 
grew out, and by the middle of the afternoon the object had become one 
of the ordinary stemmed prominences, much like Fig. 64. 

But obviously the thing is very unusual, for in more than twenty 
years of observation I have encountered the phenomenon only three 
times. 



222 THE SUN. 

present in the air, but invisible until some local cooling 
or change of pressure causes its condensation. These 
prominences are, therefore, formed by some local heat- 
ing or other luminous excitement of hydrogen already 
present, and not by any transportation and aggregation 
of materials from a distance. The precise nature of 
the action which produces this effect it would not be 
possible to assign at present ; but it is worthy of note 
that the spectroscopic observations made during eclipses 
rather favor this view, by showing that hydrogen, in a 
feebly luminous condition, is found all around the sun, 
and at a very great altitude — far above the ordinary 
range of prominences. 

Indeed, in most cases the forms and changes of 
this class of prominences so closely resemble our own 
terrestrial clouds that one is almost forced to believe 
that they are surrounded by, and float in, a medium 
which does not greatly differ from themselves in den- 
sity, though it is not visible in the spectroscopic mode 
of observation. 

ERUPTIVE PROMINENCES. 

Tlie eruptive prominences are very different — much 
more brilliant and much more vivacious and interesting. 
They consist usually of brilliant spikes or jets, which 
change their form and brightness very rapidly. For the 
most part they attain altitudes of not more than 20,000 
or 30,000 miles, but occasionally they rise far higher than 
even the largest of the clouds of the preceding class. 
Their spectrum is very complicated, especially near 
their base, and often filled with bright lines, those of 
sodium, magnesium, barium, iron, and titanium, being 
especially conspicuous, while calcium, chromium man- 
ganese, and probably sulphur, are by no means rare, 



THE CnROMOSPHEKE AND THE PROMINENCES. 223 

Scale, 75,000 miles to the inch. 



Fig. 06. 



Fig. G9. 





Peominence as it appeared at half-past 

TWKLVE o'clock, SEPTEMBER 7, 1871. 

Fig. 70. 



Vertical Filame>ts. 
Fig. 67. 





as the above appeared half an hour later, 
when the up-rushing hydrogen attained a 
Height of more than 200,000 Miles. 

Fig. 71. 




Flames. 



Spot near the Sun's Limb, with accompanying 
Jets OF Hydrogen, as seen October 5, 1871. 



224 THE SUN. 

and for this reason Secchi calls them metallic promi- 
nences. 

They usually appear in the immediate neighborhood 
of a spot, never occurring very near the solar poles. 
Their form and appearance change with great rapidity, 
so that the motion can almost be seen with the eye — an 
interval of fifteen or twenty minutes being often suffi- 
cient to transform, quite beyond recognition, a mass of 
these flames fifty thousand miles high, and sometimes 
embracing the whole period of their complete develop- 
ment or disappearance. Sometimes they consist of 
pointed rays, diverging in all directions, like hedgehog- 
spines. Sometimes they look like flames ; sometimes 
like sheaves of grain ; sometimes like whirling water- 
spouts, capped with a great cloud; occasionally they 
present most exactly the appearance of jets of liquid 
fire, rising and falling in graceful parabolas ; frequently 
they carry on their edges spirals like the volutes of an 
Ionic column; and continually they detach filaments 
which rise to a great elevation, gradually expanding 
and growing fainter as they ascend, until the eye loses 
them. Our figures present some of the more common 
and typical forms, and illustrate their rapidity of change, 
but there is no end to the number of curious and inter- 
esting appearances which they exhibit under varying 
circumstances. 

The velocity of the motions often exceeds a hundred 
miles a second, and sometimes, though very rarely, 
reaches two hundred miles. That we have to do with 
actual motions, and not with mere change of place of a 
luminous form, is rendered certain by the fact that the 
lines of the spectrum are often displaced and distorted 
in a manner to indicate that some of the cloud-masses 
are moving either toward or from the earth (and, of 



THE CHROMOSPHERE AND THE PROMINENCES. 225 

course, tangential to the solar surface) with similar 
swiftness. 

Fig. Y2 is a representation of a portion of the spec- 
trum of a prominence observed at Sherman on Angust 
3, 1872, an observation to which allusion was made in 

Fig. 72. 




the preceding chapter. The F-line, at 208 of the scale, 
must be imagined as blazingly brilliant, and fainter 
bright lines appear at 203*2, 208*8, 2094, and 212*1 (the 
scale is Kirchhoff's), while two bands of continuous spec- 
trum, produced probably by the compression of the gas 
at the points of maximum disturbance, run the whole 
length of the figure. At the upper point of disturbance 
F is drawn out into a point reaching to 207*4: of the 
scale, and indicating a velocity of 230 miles a second 
away from us ; at the lower point it extends to 208*7, 
and indicates a velocity of about 250 miles per second 
toward us. It was very noticeable that this swift 
motion of the hydrogen did not seem to carry with it 
many other substances which were at the time repre- 
16 



226 THE SUN. 

sented in the spectrum by their bright lines; mag- 
nesium and sodium were somewhat affected, but barium 
and the unknown element of the corona were not. 

When we inquire what forces impart such a velocity, 
the subject becomes difficult. If we could admit that 
the surface of the sun is solid, or even liquid, as Zollner 
thinks, then it would be easy to understand the phe- 
nomena as eruptions, analogous to those of volcanoes 
on the earth, though on the solar scale. But it is next 
to certain that the sun is mainly gaseous, and that its 
luminous surface or photosphere is a sheet of incandes- 
cent clouds, like those of the earth, except that water- 
droplets are replaced by droplets of the metals ; and it 
is difficult to see how such a shell could exert sufficient 
confining power upon the imprisoned gases to explain 
such tremendous velocity in the ejected matter. 

Possibly the difficulty may be met by taking account 
of the enormous amount of condensation which must be 
going on within the photosphere. To supply the heat 
which the sun throws off (enough to melt each minute 
a shell of ice nearly fifty feet thick over his entire sur- 
face) would require the condensation of enough vapor 
to make a sheet of liquid six feet thick in the same time 
— supposing, that is, the latent heat of the solar vapors 
not greater than that of water vapors. This, of course, 
is uncertain, but, so far as we know, very few if any 
vapors contain more latent heat than that of water, and 
we may therefore consider it roughly correct to estimate 
the continuous production of liquid as measured by the 
quantity named. Now, on the surface of the earth a 
rain-storm which deposits two inches in an hour is very 
uncommon — in such a storm the water falls in sheets. 
If we admit, then, that any considerable portion of the 
sun's heat is due to such a condensation of the solar 



Jl 



THE CHROMOSPHERE AND THE PROMINENCES. 227 

vapors, it is easy to see that the quantity of liquid pour- 
ing from the solar clouds must be so enormous that the 
drops could not be expected to remain separate, but 
will almost certainly unite into more or less continuous 
masses or sheets, between and through which the gases 
ascending from beneath must make their way. And, 
since the weight of the vapors which ascend must con- 
tinually equal that of the products of condensation 
which are falling, it is further evident that the upward 
Currents, rushing through contracted channels, must 
move with enormous velocity, and therefore, of course, 
that the pressure and temperature must rapidly increase 
from the free surface downward. It would seem that 
thus we might explain how the upper surface of the 
hydrogen atmosphere is tormented by the up-rush from 
below, and how gaseous masses, thrown up from be- 
neath, should, in the prominences, present the appear- 
ances which have been described. I^or would it be 
strange if veritable explosions should occur in the quasi 
pipes or channels through which the vapors rise, when, 
under the varying circumstances of pressure and tem- 
perature, the mingled gases reach their point of combi- 
nation ; explosions which would fairly account for such 
phenomena as those represented in Figs. 69 and 70, when 
clouds of hydrogen were thrown to an elevation of more 
than 200,000 miles with a velocity which must have 
exceeded at first 200 miles per second, and very prob- 
ably, taking into account the resistance of the solar 
atmosphere, may, as Mr. Proctor has shown, have ex- 
ceeded 500 ; a velocity suflicient to hurl a dense material 
entirely clear of the power of the sun's attraction, and 
send it out into space, never to return. 

And yet such velocities so far exceed those with 
which we are familiar here that it is not strange at all 



228 THE SUN. 

that there should be reluctance to admit them, and at* 
tempts to substitute for such motions of material masses 
the motion of mere forms and the swift transference 
of regions of luminosity through gases themselves at 
rest — just as when a flash runs from one end to the 
other of a long train of gunpowder, or a suddenly 
kindled flame flies up through a chimney. In many 
respects such conceptions perfectly represent the facts 
— prominences apparently at rest might be like water- 
falls or gas-flames — mere stationary forms made up 
from a steady succession of material particles ; and the 
swiftly moving ones, so far as appearances go, might be 
flashes traveling swiftly through extensive masses of gas 
comparatively motionless. If such a view is tenable, then 
we might imagine, as Brester has done, that the sun is 
quiescent and serene, composed of overlying strata of 
different density, each in a state of stable equilibrium, 
such that any considerable vertical motions are impossi- 
ble, and horizontal disturbances soon checked : what 
look to us like fiery flames and furious commotions are 
then only like the auroral flickerings in our own atmos- 
phere. 

But to this view the one conclusive objection, unless 
it can be evaded, is the fact that the lines in the spec- 
trum testify to swift motions in the line of vision — that 
masses of hydrogen and helium, of iron-vapor and cal- 
cium, are shown to be moving toward or from us with tre- 
mendous velocity. Brester therefore maintains, and oth- 
ers with him, that though unquestionably the motion of j 
a luminous mass of matter toward or from the observer] 
will produce such line-displacements as are observed, w^e 
are not shut up to that as their only explanation. He 
maintains that the motion of a mere luminous /brm 
would produce the same effect : that if a train of pow-. 






THE CHROMOSPHERE AND THE PROMINENCES. 229 

der, for instance, were laid straight away from us for 
a distance of ten miles, then if it were Hghted at the end 
nearest us and the flash reached the other end in ten 
seconds, the spectrum of the traveling flash would indi- 
cate a receding velocity of one mile a second. There 
is, however, no evidence to support such a doctrine. 
No theoretical reason can be assigned — at least none has 
been so far as we know — w^hy the phases of the light- 
waves issuing from the flash at each point in its ten- 
mile course should reach the observer with the same 
regular retardation as in the case of a luminous ball 
moving over the same path w4th the same speed : and 
unless this condition is observed, or something essen- 
tially equivalent, Doppler's jDrinciple has no application. 
As for experimental evidence, none exists as yet, nor 
do we know of any proposed method by which the hy- 
pothesis can be tested. 

Still a different and very curious theory of the solar 
constitution has been lately proposed by Schmidt, of 
Stuttgart, and a good deal discussed (rather favorably, 
too, on account of its mathematical interest) by various 
writers. It amounts to this : that the sun is a great 
globe of heated transparent gas, much denser in the 
center, and the apparent definiteness of outline is due 
to the curious refraction of the light in such a medium. 
The rays from points behind the sun's center, according 
to this theory, reach us from all around the limb ; pho- 
tosphere and chromosphere are an optical jumble of 
rays from widely different points within the globe, and 
most of the phenomena we see on and about the solar 
surface are purely optical, like halos, rainbows, and 
mirages. 

it is probably sufficient to point out that a gaseous 
globe which contains in itself quantities of metallic vapor 



230 THE SUN. 



1 



can not remain wholly gaseous for any length of time. 
In its outer regions, where it is exposed to the cold of 
space, condensation must inevitably take place, incan- 
descent clouds must gather, and a " photosphere " must 
form — it must " clothe itself with light as with a gar- 
ment." The theory can apply only to a mass composed 
wholly of " permanent " gases — those that will not turn 
to liquid or solid even at the lowest temperatures to 
which they are anywhere exposed. It may be that in 
the planetary nebulae we have such bodies. 

PROMINENCE PHOTOGRAPHY. 

As far back as 1870 attempts were made by the 
writer to photograph the prominences, and a partial 
success was reached. A little camera carrying a sensi- 
tized microscope slide was fitted to the spectroscope 
figured on page 202, and with a four-minute exposure aft 
distinct impression of a prominence was obtained. Ther 
Jiydrogen line employed was ^, (H7). It was in the days 
of the wet-plate collodion process, and the necessity of 
so long an exposure made it certain that it would not 
be worth while then to follow up the matter. But the 
introduction of the modern dry-plate has changed all 
that. The subject was resumed in 1884 and 1890 almost 
simultaneously by Deslandres, in France, and George 
E. Hale, in Chicago. By 1891 it had become possible 
to produce very fair pictures of moderate-sized prom- 
inences by using the H or K lines with a powerful 
spectroscope, and putting a photographic plate in place 
of the eye.- If the slit is narrow we get merely the 
double reversal of H and K, as shown in Fig. 73. (It 
is worth while to notice, in passing, the hydrogen line 
(He) which is so near to H. For years it had been a 
puzzle why in the spectra of stars of the so-called " first " 



THE CHROMOSPHERE AND THE PROMINENCES. 231 

class, like Vega, H should be conspicuous and K miss- 
ing. The discovery of this hydrogen line by Ames 
solves tlie problem. In Vega's spectrum H is the hydro- 
gen, not the calcium band.) To return : if now we sim- 
ply open the slit as far as can safely be done, we get the 




DOUBLE-REVEKSAL OF 11 AND K LlNES. 

image of the prominence in each of the two bands, as 
in Fig. 74. An exposure of live seconds is abundant. 
Indeed, at Princeton, by using isochromatic plates with 
an exposure of several minutes, we have been able to 
photograph prominences even in the C-line (Fig. 75). 
But this requires extremely accurate adjustment of the 
clockwork of the telescope and careful manipulation. 

With this open-slit arrangement, however, we are 
limited to prominences that are not very large ; nor is 
the definition very perfect. 

These difficulties may be avoided by adopting an 
arrangement long ago suggested by Janssen and others 



232 



THE SUN. 



in tlie early days of prominence observation. Tlie 
spectroscope is fitted up with a second slit at the eye 



Fig. 74. 




Tkominence in H and K Lines. — (Fkom Photograph ) 

end of the view-telescope, and in some forms of the ap- 
paratus both the collimator slit and the other are free 
to slide back and forth in the focal plane and length- 

FiG. 75. 




PllOMINENCE PlIOTOCiRAPllED IN C-LlNE. 

wise of the spectrum. Beginning with the slits each at 
the center of its slide, suppose the prism or grating to 



THE CHROMOSPHEKE AND THE PROMINENCES. 233 

be so adjusted as to bring the K line into position to be 
seen through the second slit ; if now we slide the colli- 
mator slit, the K line will move away from the second 
slit, and to keep it in view this one wdll have to slide 
also. This can be automatically affected : the two 
sliders that carry the slits can be connected mechanic- 




Hale's Spectro-Heliograph. 



ally in various ways so that their motions shall exactly 
correspond ; and if we add a pliotographic plate and its 
accessories we have the so-called " spectro-heliograph." 

Fig. Y6 is from a photograph of Professor Hale's 
instrument, as used in 1892 at the Kenwood Astro- 
Physical Observatory, in Chicago, 



234 THE SUN. 

In order to pliotograpli a prominence the telescope 
is so directed as to bring the base of the prominence to 
the collimator slit — the slit being tangential to the sun's 
limb. The clockwork of the equatorial will keep it 
there if properly adjusted. Then the collimator slit is 
made to slide smoothly and gradually upward to the 
top of the prominence (by a hydraulic apparatus in 
Hale's instrument), and at the same time the other slit 
travels in front of the sensitive plate, so that this re- 
ceives, one after the other, the imprints of all the suc- 
cessive sections of the prominence. As an example of 
a y3rominence photograph so made, we give Fig. 77, 
which was taken March 25, 1895. The maximum eleva- 
tion was very nearly 281,000 miles. The vertical dark 
streaks are " dust-lines " caused by motes in the slit or 
roughness of its edges; the streaks at right angles to 

Fi-j. T7. 




Prominence op March 25, 1895.— Spectro-Heliograph Photograph. 

tiiese are due to slio;ht unsteadiness in the slidino^ mo- 
tion, produced by the hydraulic "clepsydra." 

If the image of tlie sun itself is covered by anj 
opaque disk of exactly tlie right size, then the slits mayj 



THE CHROMOSPHERE AND THE PROMINEKCES. 235 

be made to traverse the whole chromosphere, rather 
slowly, at a single journey, and we shall get at one ex- 
posure a picture of the whole array of prominences 

Fig. 7S. 




SpECTEO-HeLIOGKAPH PnOTOGUAPH OF THE ENTIRE ClIROMOSPHEEE. 

surrounding the sun at that time. Figs. 78 and 79 were 
made in this way at Kenwood Observatory, though we 
can not give the exact date. 

If after making such an exposure the screen that 
covered the sun's disk is removed, and the collimator 
slit is made to retrace its path (swiftly this time), we 
shall get, not only the chromospheric ring with its out- 
lying prominences, but the whole surface of the snn 
itself as seen by the monchromatic, ''K-Iine," light. 
The faculous regions come out with special emphasis. 
The reader w^ill recall the pending discussion w^th re- 
gard to them between Hale and Deslandres, referred to 
on page 109. Fig. 31"^ on that page is an example of 
this sort — also from Mr. Hale. 

The "two-slit" arrangement admits of various mod- 
ifications : in one the spectroscope and its slits are fixed, 
the image of the sun is allowed to drift over the colli- 



236 THE SUN. 

mator slit by the diurnal motion, and the photographic 
plate is drawn along at the same rate, and in the same 
direction, by a suitably adjusted clockwork. In this 
case the large telescope that forms the sun's image is 
usually also lixed in a horizontal position, and the sun's 
rays are directed into it by a plane mirror, as in the 
American transit of Venus apparatus. The instrument 
of Deslandres is arranged in this manner, and with it he 
has obtained all the results that Hale has reached. 

Fig. 79. 




SPECTKO-IlELio'JRAPn FnoTOGRArn OF Prominences. 

In the great spectro-heliograph now building under 
Mr. Hale's direction, to be used with the gigantic 
forty-inch equatorial of the Yerkes Observatory, the 
slits will be fixed in the spectroscope, but the whole spec- 
troscope will be so arranged in the framework which 
attaches it to the equatorial that it can be moved bodily 
across the seven-inch image of the sun, while the plate- 
holder remains fixed. 

Students of solar physics await with great interest 
the outcome of the new methods and apparatus. 



4 



CHAPTER yiL 

TEE CORONA. 

General Appearance of the Phenomenon. — Various Representations. — 
Eclipses of 1857, 1860, 1867, 1868, 1869, 1871, 1878, 1882, 1889, 
and 1893. — Proof that the Corona is mainly a Solar Phenomenon. — 
Brightness of the Corona. — Connection with Sun-Spot Period. — ^Spec- 
trum of the Corona. — Application of the Analyzing and Integrating 
Spectroscopes. — Polarization. — Evidence of the Slitless Spectroscope 
as to the Constitution of the Corona. — Changes and Motions in the 
Corona. — Its Form and Constitution, and Theories as to its Nature 
and Origin. 

A TOTAL eclipse of the sun is unquestionably one of 
the most impressive of all natural phenomena, and the 
corona, or aureole of light, which then surrounds the 
sun, is its most impressive feature. On such an occa- 
sion, if the sky is clear, the moon appears of almost 
inky darkness, with just sufficient illumination at the 
edge of the disk to bring out its rotundity in a strik- 
ing manner. It looks not like a flat screen, but like 
a huge black ball, as it really is. From behind it 
stream out on all sides radiant filaments, beams, and 
sheets of pearly light, which reach to a distance some- 
times of several degrees from the solar surface, forming 
an irregular stellate halo, with the black globe of the 
moon in its apparent center. The portion nearest the 
sun is of dazzling brightness, but still less brilliant than 
the prominences, which blaze through it like carbun- 
cles. Generally this inner corona has a pretty uni- 
form height, forming a ring three or four minutes of 



238 THE SUN. 

arc in width, separated by a somewhat definite outh'ne 
from the outer corona, which reaches to a much greater 
distance, and is far more irregular in form. Usually 
there are several " rifts," as they have been called, like 
narrow beams of darkness, extending from the very 
edge of the sun to the outer night, and much resem- 
bling the cloud-shadows which radiate from the sun 
before a thunder-shower. But the edges of these rifts 
are frequently curved, showing them to be something 
else than real shadows. Sometimes there are narrow, 
bright streamers, as long as the rifts, or longer. These 
are often inclined, occasionally are even nearly tangen- 
tial to the solar surface, and frequently are curved. On 
the whole, the corona is usually less extensive and brill- 
iant over the solar poles, and there is a recognizable 
tendency to accumulations above the middle latitudes, 
or spot-zones; so that, speaking roughly, the corona 
shows a disposition to assume the form of a quadrilat- 
eral or four-rayed star, though in almost every individual 
case this form is greatly modified by abnormal streamers 
at some point or other. 

Unlike the chromosphere, which seems first to have 
been observed, as was mentioned in the previous chap- 
ter, only a little more than a century ago, the corona 
has been known from antiquity, and is described by 
Philostratus and Plutarch in almost the same terms we 
should ourselves employ. And yet our knowledge of 
it remains very limited. The chromosphere and promi- 
nences we can now reach and study, comparatively at 
our leisure, by the help of the spectroscope ; but the 
corona is still inaccessible, except during the short and 
precious moments of a total eclipse — in all, not more 
than a few days in a century — so that our knowledge 
of its cause and nature can grow but slowly at the best. 



THE CORONA, 239 

The character of the phenomenon is such also as 
to make its accurate observation exceedingly diflScult ; 
slight differences in the transparency of the atmosphere, 
in the sensitiveness of the observer's eye, a preoccupa- 
tion of the mind by some feature which first happens 
to strike the attention, or a peculiarity in the manner 
of representing what one sees, will often make the de- 
scriptions and drawings of two observers, side by side, 
so discrepant that one would hardly imagine they could 
refer to the same object. For instance, in 1870, two 
naval officers on the deck of the same vessel made 
drawings of the corona, one of which represented it as 
a six-rayed star, while the other showed it as composed 
of two ovals crossing at right angles. In 1878 the 
writer, on comparing notes immediately after the 
eclipse with other members of his party, found that 
about half of them saw the corona principally extended 
to the east and west, while the other half, himself 
among them, were just as positive that it brushed out 
mainly to the north and south. The photographs, and 
other data since collected, show that the principal exten- 
sion was undoubtedly along the east-and-west line, but 
that there were much better outlined streamers, though 
shorter and less brilliant, directed from the solar poles. 
Some eyes were more impressed by definiteness of form, 
others by size and luminosity. 

Obviously, conclusions must be drawn from ocular 
impressions only with the greatest caution. Photo- 
graphs are, of course, more to be trusted, as far as they 
go ; but, even with them, a slight difference in the 
sensitiveness of the plate, in the exposure, or in the 
development, will make a great difference in the result- 
ing picture. Neither can any photograph ever bring 
out everything which is visible to the eye. An ex- 



240 



THE SUN. 



posiire, sufficient to exhibit well the fainter details, will 
spoil the brighter features, and vice versa. Moreover, 
it may, and not seldom does, bring ont features that the 
eye can not see because their light is mainly ultra-violet. 
We can do no better than to refer one, who is curious 
to see how various are the representations of this won- 
derful object, to Mr. Eanyard's magniiicent work upon, 
the observations made during total solar eclipses, pub- 
lished as Volume XLI of the '' Memoirs of the Royal 
Astronomical Society of Great Britain." In it he has 
reproduced nearly a hundred different drawings and 
photographs of the corona, as seen during the eclipses 
since 1850. The steel engravings of the eclipses of 



Fig. 80. 




OOKONA AS OBSERVED BY LlAlS IN 1857. 



THE CORONA. 241 

1870 and 18Y1, based upon tlie photographs then made, 
are among the most accurate and beautiful representa- 
tions of the corona anywhere to be found. We have 
copied a few of his woodcuts, which give an idea of 
the more remarkable features of the phenomenon, and 

Fig. 81. 




CoBONA OF I860.— Secchi. 

exhibit the differences between its character and ap- 
pearance on different occasions ; we have added also a 
picture of the corona as seen in 1878, in which we have 
combined the sketches of several observers with our 
own impressions. Woodcuts, however, are not com- 
17- 



242 THE SUN". 

petent to bring out tlie peculiar filmy, nebulous charac- 
ter of many of the details, which can be fairly repre- 
sented only by steel engraving. 

Fm. 82. 




Corona of 1^60. — Tempkl. 

The drawing of Liais, Fig. 80, shows the " petal ''- 
like forms which have been noticed in the corona at 
other times, but seem to have been especially prominent 
in the eclipse of 1857. The tigures of the corona of 
1860, by Secchi and Tempel (Figs. 81, 82), show how: 
widely observ^ers only a few miles apart will differ in 
their impressions. 



I 




THE CORONA. 



243 



The drawing of Groscli in 1867 (Fig. 83) is interest- 
ing in comparison with that of 1878, as showing the 
state of the corona at two similar times of sun-spot mini- 
mum. The long extensions of faint illumination in the 
direction of the sun's equator and the short but vivid 
brushes in the polar regions are notable in both. 

Fig. Sa. 




COEONA OF 1S67. — GROaCH. 



Bullock's picture of the eclipse of 1868 (Fig. 84) 
shows a larger and more irregular corona than usual. 
The drawing of Schott (Fig. 85), on the other hand, 
shows the corona of 1869 much smaller and more brill- 



244 



THE SUN. 



iant than ordinary, and the writer can vouch for it as 
giving pretty accurately the impression which he him- 
self received at the time. 

Many of our readers^ no doubt, have seen a much 
more impressive picture of the same corona, made by 
Mr. Gihnan at Sioux City, and published in the eclipse 
report of the United States Naval Observatory (repro^ 

Fig. SA 




CoiiONA OF 1868.— Bullock. 



duced in Mr. Proctor's " Sun "). It shows an exten- 
sive system of rifts and rays, which, if real objects, 
escaped the notice of most observers — their visibility. 



THE CORONA. 245 

perhaps, depending on the state of the atmosphere, 
which is described as slightly hazy, but very steady, at 
Mr. Gihnan's station. 

The drawings of Captain Tupman and Mr. Foe- 
nander (Figs. 86, 87) are interesting for comparison 

Fig. 85. 




Corona of 18G9.— Schott. 



with each other and with the photographs of the same 
echpse (Fig. 88); and that of the eclipse of 1878 (Fig. 
89) is remarkable on account of the enormous exten- 
sion of the faint brushes of nebulosity which were 



246 



THE SUN. 



1 



traced to a distance of 6° or 7° from the sun by Pro- 
fessors Langley, Abbe, and Newcomb. 

To these pictures of the corona that appeared in our 
first edition, we add three others that seem especially 
worthy of reproduction. 

Fig. 90 is from a steel engraving which combines 
the photographs of the Egyptian eclipse of May 17, 

Fig. S6. 




Corona op 1871.— Captain Tupman. 



1882. Like Figs. 88 and 92, it is a typical "spot-maxi- 
mum corona." Attention is called to the little comet 



THE COKONA. 247 

in the corner of the engraving : it was seen only dur- 
in;^ the eclipse, but seems to have been a precursor of 
the great comet that appeared the following autumn. 

Fig. 87. 




COBONA OF 1871. — FOENANDER. 



Fig. 91 is a direct reproduction from a beautiful 
negative of the eclipse of January 1, 1889, made by 
Mr. Burckhalter, of Oakland, CaL, with an exposure of 
one second. It is a typical " spot-minimum " corona, 
the polar streamers being extremely fine, and the equa- 
torial extension enormous ; this latter, however, is better 
brought out on negatives of longer exposure. 



248 



THE SUN. 



Fig. 92 is from what, on the whole, is the finest 
photograph ever yet obtained of the corona. It was 
made on April 16, 1893, at Mina Bronces, in Chili, at 



FtG. 88. 




Corona of 1871.— From PnoTOGRAPns of Mr. Davis, 

an altitude of G,600 feet, by Professor Schaeberle, with 
a photoheliograph telescope of 40 feet focal length and 
5 inches aperture ; the sun's disk in the original is about 
4 inches in diameter. 

One of the first questions which suggests itself with 
reference to the corona relates to its location : is it a 
phenomenon of the sun, of the moon, or of our own 



I 



THE COnONA. 



249 



atmosphere; or is it perhaps a mere optical effect, like a 
rainbow or a halo ? If its seat is in the earth's atmos- 
phere, it is of course an affair of little magnitude or 
importance ; if, on the other hand, it is really at the 
sun, it must be an object of enormous dimensions and 
of cosmical significance. 

Kepler, and many astronomers after liim, attributed 
it to the atmosphere of the moon, and this continued, 
perhaps, to be the most generally accepted explanation 

Fig. so. 




Corona of 1878.— From Combination of Various Drawings. 



until the early part of the present century, when it was 
shown by many incontestable considerations that the 
moon possessed no atmosphere to speak of ; certainly 



250 THE SUN. 

none which could account for the observed facts. From 
tliis time until 1869 the weight of opinion seems to 
have been rather in favor of a terrestrial or purely- 
optical origin for the corona, though some (Professor 
Grant, among others, in 1852, in his '' History of Physi- 
cal Astronomy ") considered it more probable that the 
solar atmosphere is the real cause. 

The question was first settled in 1869 by the obser- 
vations of Professor Harkness and the writer, who, 
independently, found the spectrum of the corona to be 
characterized by a bright line in the green. It was 
identified by the writer, whose spectroscope was very 
powerful for the date, as the " 1,474 " line on Kirch- 
hoff's map of the solar spectrum, then generally used 
for reference. The existence of this bright line demon- 
strates the presence, in the corona, of incandescent gas, 
and this of course can only be near the sun. Some 
doubt was cast upon the observations at first, but they 
have since been fully confirtoed ; and in 1871 a differ- 
ent and more simple proaf was added. Photographs, 
taken at stations which were separated by several hun- 
dred miles, in India and Ceylon, showed precisely the 
same details of coronal form and structure, and are, by 
themselves considered, suflicient to demonstrate that the 
main features of the phenomenon are independent of 
our terrestrial atmosphere and the accidents of the lunar 
surface. Of course, it is not meant to afiirm that our 
own atmosphere has no part in the phenomenon, but its 
role is only secondary. As has been pointed out by 
Mr. Proctor, the observer at the middle of an eclipse is 
in the center of an enormous shadow, generally from 
fifty to a hundred miles in diameter. If we grant that 
the air retains some sensible density and power of light- 
reflection, even at an altitude of a hundred miles, and 



THE CORONA. 



251 



assume for the shadow a radius of only twenty miles, 
no particle of air illuminated by sunliglit could, under 
these circumstances, be found within 11° of the sun's 
apparent place in the sky. If there were no corona 
truly solar in its origin, there would therefore be around 
the moon a circle of intense darkness, 23° at least in 
diameter : at the edge of thi^ circle a faint illumination 
would begin, forming a luminous ring, something like 
a halo, outside of which the sky would be lighted by 
rays from an only partially hidden sun. Of course, this 

Fig. 90. 




CoEONA OF 18S2. Egypt. 



dark " hole in the sky " would be concentric with the 
sun and moon only at the moment when the eclipse 
was central. In the actual state of things, the portion 
of the sky in the neighborhood of the sun is, of 
course, illuminated by whatever appendages of the sun 



252 THE SUN. 

remain unhidden by the moon, and it is this faint illumi- 
nation, derived from the corona and prominences, which 
gives to the lunar disk its apparent solid rotundity. 

We have spoken of this illumination as faint, but 
generally it is considered to be much stronger than that 
of the full moon, though there is some difference of 
opinion on the matter. There is no doubt that in many 
cases there is abundant light for reading a watch-face, 
even at the middle of the totality ; the writer, in 1869, 
found no use for a lantern in making notes or in read- 
ing a micrometer-head. But undoubtedly a large por- 
tion of this light is derived, not from the corona, but 
from the illuminated air ; for, though the observer him- 
self is in darkness, he has in sight all around the horizon 
a sunlit atmosphere.^ 

Undoubtedly there is a great difference between 
different eclipses in respect to the obscurity. The 
brilliance of the lower part of the corona — a narrow 
ring close to the limb of the sun — ^is dazzling ; but the 
liglit falls off very rapidly. In an eclipse of long du- 
ration, therefore, when the moon's apparent diameter 
considerably exceeds the sun's, the brighter portion of 
the corona will be covered, and the liglit will be much 
less than in an eclipse occurring when the difference 
between the diameters of the sun and moon is only 
small. 

At the eclipse of 1869 an attempt was made to 

* This is specially obvious if the sky is covered with clouds of medi- 
um density. In August, 188 7, the writer had the misfortune to occupy 
a station (about 120 miles northeast of Moscov), where it was wholly 
overcast, and a misty rain was falling much of the time during the 
eclipse. At the middle of the eclipse the darkness was hardly greater 
than in a heavy thunderstorm : the moment when " totality " began 
could not be determined with any accuracy at all, and its close was 
doubtful by some seconds. Fine print could be read all the time. 



THE CORONA. 253 

measure tlie darkness of the totality, as compared with 
that of niglit. The obscurity proved to be so much 
deeper than liad been expected, that the ingenious in- 
strument whicli Professor Eastman had devised for the 
purpose turned out inadequate to deal with it exactly. 
The apparatus consisted of a tube about ten inches long 
and two and a half in diameter. At the bottom of this 
was painted a small white star of five points, with a 
black dot in the center, and a black ring around it. 
The other end of the tube was closed with a so-called 

Fig. 91. 




Corona of January, 1889. — Bijrckhardt. 

" cat's-eye," a square opening, the size of which can be 
varied at will, by moving two slides with a micrometer- 
screw, or rack and pinion. 

A small tube, attached obliquely to the large one, 
like a teapot-nose, allow^ed the observer to look at the 
star, and the amount of light from the sky was then 
measured by opening or shutting the slides until the 
dot and ring in the center of the star just ceased to be 



254 THE sux. 

visible. Not only did the ring and dot become invisible 
with the whole aperture of the cat's-eye, but the star 
itself was invisible during the totality. Professor East- 
man, on the whole, concluded that the general darkness 
was on this occasion about the same as an hour or so 
after sunset, when third-magnitude stars first become 
visible. The instrument was pointed at the zenith, 
however, and not at the corona, so that it gave no direct 
determination of the coronal Hght. Neither do the ob- 
servations of Mr. Ross, in 1870 (by which the general 
illumination was compared with the light from a candle), 
answer the purpose any better. And substantially the 
same is true of the observations made at subsequent 
eclipses ; especially in 1886 and 1889. 

One or two attempts have been made to compare 
the shadow cast by the corona with that produced by a 
candle ; but the coronal shadow has always been so 
masked by the general aerial illumination as to defeat 
the observation. One astronomer only, so far as known 
to the writer, has made an estimate of the coronal light 
based on anything like a scientific foundation. Belli, 
in 1842, found that the corona seemed to him to give 
as much light as a candle at a distance of 1'8 metre. 
He was short-sighted, so that an object like a candle 
appeared to him as a confused patch of light, and it 
was by taking advantage of this defect in his vision 
that he was able to effect the comparison, which must, 
liowever, have been only very rough. Two weeks later 
lie compared, in the same way, the full moon, at the 
same altitude, with a similar candle, and thus found 
that the light of the corona was less than one sixth that 
of the moon. This comparison, how^ever, is so unsatis- 
factory in its details that no great weight can be allowed 
it, and it must, perhaps, be still considered an open 



THE CORONA. 



255 



question whether the light of the corona is brightefr or 
not than that of the moon. 

The lower portions of the coronal ring, close to the 
sun, are usually much too bright to be looked at com- 
fortably with a telescope unprovided with a shade-glass ; 
we have on this point the testimony of Biela, Struve, 
Eanyard, and others. Moreover, at a transit of Venus or 



Fig. 92. 




COEONA OF 



Mercury under favorable circumstances, the black disk 
of the planet becomes visible before it reaches the sun. 
Janssen thus saw Venus in 1874, and Langley, Mercury 
in 1878. Of course, this implies behind the planet a 
background of sensible brightness in comparison with 
the illumination of our atmosphere. It is generally 
considered that a difference of one sixty-fourth in the 



25.6 TEE SUN. 

brightness of two adjacent portions of a surface is the 
smallest quantity perceptible by the eye, and, if so, the 
corona must be more than one sixty-fourth as bright 
as the aerial illumination at the edge of the sun's disk. 
At an eclipse, also, the corona is sometimes seen several 
seconds, or even minutes, before the beginning and 
after the end of totality. Petit, in 1860, reports seeing 
it twelve minutes (sic) before the disappearance of the 
sun, and Lockyer, in 1871, continued to see it for three 
minutes after the sun's reappearance. But, as has been 
said before, the light falls off very rapidly, and the out- 
er portions of the corona are of the faintest nebulosity. 
It is greatly to be desired that, at the next eclipse, some 
careful photometric measurements should be made. 

Apart from the difference in the amount of light 
at different eclipses, due to the variation in the moon's 
diameter, there is a strong probability that the corona 
itself changes considerably in brightness and extent 
from year to year. In 1878 it was the general verdict 
of the numerous observers, who had also seen the eclipse 
of 1869, that the corona was much less brilliant than 
on the former occasion. Still, several observers of 
deservedly high reputation hold a precisely contrary 
opinion. The corona of 1878 was unquestionably the 
more extensive. 

Of course, the known facts as to the periodicity of 
sun-spots, and the sympathy between them and the 
prominences, make it antecedently probable that a cor- 
responding variation will be found in the corona. 

In the eclipses of 1867, 1878, and 1889, all of which 
occurred near the sun-spot minimum, the corona was 
characterized by very long, faint equatorial extensions, 
with distinctly defined diverging polar rays : on the 
other hand, in 1870, 1882, and 1893, the equatorial 



THE CORONA. 257 

wings and polar rays were much less striking, the corona 
was more nearly circular, and its principal development 
was over the sun-spot zones. Figs. 89 and 92 may be 
taken as nearly typical. 

In the eclipse of 1878, which occurred at a sun-spot 
minimum, the spectroscopic peculiarities of the corona 
were also greatly modified. The bright line, which is 
its principal characteristic, became so faint that many 
observers missed it altogether. 

This bright line, as has been said before, was first 
recognized as coronal at the eclipse of 1869. It had 
been seen reversed in the spectrum of the chromosphere 
a few weeks previously, both by Mr. Lockyer, and, in- 
dependently, by the writer, who, however, did not know 
of the earlier observation until some time after the 
eclipse. In the ordinary solar spectrum it appears as a 
fine, dark line at 1,474 of Kirchhoffs scale, or 5,316*9 
of Rowland's — a line in no way conspicuous as com- 
pared with liundreds of others, and barely visible with 
a single-prism spectroscope. With a spectroscope of 
high dispersion it was found, in 1876, to be closely 
double, the upper (more refrangible) component being 
slightly hazy, while the other is sharp and well-defined. 
The upper component is the true coronal line, and is 
always seen without much difliculty, reversed in the 
spectrum of the chromosphere. Both Kirchhofl: and 
Angstrom give the line as •belonging to the spectrum 
of iron^ a fact which was for a time very perplexing, 
since it is hardly possible that the vapor of this metal 
could really be the prevailing constituent of the corona, 
surmounting even hydrogen itself. This difficulty, how- 
ever, no longer exists, for it is now clear that the iron- 
line is the lower component of the double, its close 
proximity to the other being only accidental. The 
18 



258 THE SUN. 

figure gives a representation of the line and its sur- 
roundings, as seen in a liigh-dispersion spectroscope. 
The scale above the spectrum is that of Angstrom. 




POETION OF THE SPECTRUM NEAR THE CORONA LiNE (CC), 

as seen with an instrument of high dispersion. 

The hydrogen-Hnes and H and K also appear briglit 
in the corona-spectrum. It is, perhaps, not quite certain 
that this may not be due to reflection of the light of the 
chromosphere in our own atmosphere, but, on the vi^hole, 
probably not. The atmospheric reflection extends in- 
ward, at an eclipse, over the dark disk of the moon, as 
well as outward, and if the appearance of the hydrogen- 
lines were due simply to this reflection, they should be 
just as strong on the moon's disk as in the corona. This 
does not seem to be the case, though in 1870 the writer 
saw them plainly on the center of the lunar disk ; but 
Janssen and Lockyer agree that they are much brighter 
outside. The " 1,474 line " has been traced, by an ana- 
lyzing spectroscope, on some occasions to an elevation 
of nearly 20' above the moon's limb, and the hydrogen- 
lines nearly as far. What is important also, the lines 
were just as strong in the raiddle of a dark rift as any 



THE CORONA. 259 

where else. We shall have occasion to recur to this 
again. 

With the analyzing spectroscope the 1,474 line is 
very much feebler near the sun's limb than the hydro- 
gen-lines, i. e., taking any small portion of the corona 
near the limb, the hydrogen is much more brilliant than 
the unknown vapor which produces the other line. 
When, however, the eclipse is examined by an integrat- 
ing spectroscope,"^ the relation of brightness is reversed, 
showing that the total amount of "1A74: light" is the 
greater, and indicating either that it comes from a much 
more extensive area, or else that in the upper regions 
the hydrogen loses its brightness much more rapidly 
than the other material. 

As to the substance which produces the 1,474 line 
we have no knowledge as yet, though the name " coro- 
nium" has been provisionally assigned to it, and the 
recent probable identification of '' helium " in terrestrial 
minerals gives strong reason to hope that before very 
long we may find coronium also. It would seem that it 
must be something with a vapor density below that of 
hydrogen itself, which is incomparably the lightest of 
all bodies now known to our terrestrial chemistry. It 
can hardly be any one of our familiar elements, even in 
any allotropic modification, such as has been suggested 
by some, for, in the midst of tlie most violent disturb- 
ances which are observed sometimes in prominences and 
near sun-spots, when the lines of hydrogen, magnesium, 
and other metals are contorted and shattered by the 
swiftness of the rush of the contending elements, this 
line usually remains undisturbed, fine, sharp, and 
straight ; a little brightened, but not otherwise af- 
fected. For the present it stands (as did the helium 

* See pages 12, 73 for explanation of this term. 



260 THE SUN. 

lilies until Kamsay's discovery) an unexplained mys- 
tery.^ 

Besides this line and the hydrogen-lines, two others 
have been doubtfully reported in the greenish-yellow 
part of the spectrum. One of them seems to have been 
seen twice : first, in 1869 by the writer, and in 1870 by 
Denza, in Italy. Its place is about 5,570 of Angstrom's 
scale. Still, as one of the barium-lines, which is fre- 
quently and brilliantly reversed in the spectrum of the 
chromosphere, is not very far from this place (at 5,534), 
it is quite possible that this w^as the line seen. The 
other doubtful line (reported by the writer in 1869) w^as 
at 5,450 (Angstrom), also very near, in fact between, 
the places of two lines which are conspicuous in the 
chromosphere. 

The photographic spectrum of the corona, observed 
more or less fully at every eclipse since 1882, is full of 
detail and interest. Its most striking feature is the 
great calcium pair, H and K, but the violet and ultra- 
violet lines of hydrogen are also conspicuous, and there 
are a multitude of others less so. It is not easy, how- 

* Its frequent identification with a line in the spectrum of the aurora 
borealis, for which, unfortunately, the writer was at first mainly respon- 
sible, is a striking example of the difficulty of correcting a mistake which 
has once gained currency. A few weeks before the first discovery of this 
line in the spectrum of the corona, Professor Winlock had observed 
the spectrum of a bright aurora, and had published the position of five 
lines: one of the five positions coincides with that of the 1,474 line far 
within the limits of error probable in such an observation, and I jumped 
to the conclusion that the coincidence was exact and significant. Later 
observations soon showed that this *' line " in the aurora spectrum is not 
a Hne at all, strictly speaking, but a faint, hazy band, never to be seen 
except in unusually bright auroras, and not at all identifiable with the 
1,474 line of the corona. So far as the spectroscope goes, there is no 
indication of any connection between the corona and the aurora of the 
earth's atmosphere, though there are other facts which suggest that the 
phenomena may be to some extent similar in their nature. 



THE CORONA. 261 

ever, to discriminate between those lines that are 
truly coronal and those that belong to the chromo- 
sphere. It would be well at the first opportunity to 
obtain photographs with simple " integrating " spectro- 
scopes as well as with " analyzing " instruments. 

Besides bright lines, the corona shows also a faint 
continuous spectrum, and in this Janssen and Barker 
have observed a few of the more prominent dark lines 
of the solar spectrum — D, J, and G especially. 

This fact of course shows that while the corona may 
be in great part composed of glowing gas, as indicated 
by the bright lines of its spectrum, it also contains a 
considerable quantity of matter in such a state as to re- 
flect the sunlight — matter, probably, in the form of dust 
or fog. 

This conclusion is borne out also by the result of 
observations with different forms of polariscope, which, 
for the most part, indicate that the light of the corona 
is partially polarized in radial planes, just as it should 
be if in part composed of reflected light. We have said 
''for the most part," because there have been some 
very puzzling discrepancies between different instru- 
ments and different observers, which we have not space 
to discuss here. 

Since the corona, tlien, contains both incandescent 
gas and also matter in such a condition of mist or smoke 
as fits it to reflect light, it is an interesting question 
whether different parts of the coronal structure are com- 
posed alike of both, or whether there is a separation. 

It has been attempted to solve the question by ex- 
amining the eclipse with a so-called " slitless spectro- 
scope " — i. e., simply a prism put in front of the object- 
glass of a small telescope. If, jvith such an instrument, 
one were to look at a distant object emitting homoge- 



262 THE SUN. 

neous light (an alcohol-flame tinged with salt, for in- 
stance), one would see it precisely as if the prism w^ere 
not there, except that the refraction would change the 
apparent direction of the object. If the light were 
composed of three or four bright lines, like that from a 
Geissler tube filled with hydrogen, there would then 
appear the same number of colored images. If the 
light were like that of an ordinary candle, which gives 
a continuous spectrum, one would get merely a colored 
streak. Finally, if w^e had a source of light combining 
these different conditions, a lamp-flame, for instance, 
tinged in some parts with sodium and in others wdtli 
lithium, we should tlien have the streak of color marked 
in the yellow with a clear image of the sodium part of 
the flame, and in the red and violet with images of that 
part of the flame which was colored by lithium. 

If, then, the long rays and streamers of the corona 
were mainly composed of the gaswdiich gives the 1,4Y4 
line, we ought to see them distinctly through the prism 
on a background produced by the light from the reflect- 
ing mist. Nothing of the kind occurs, however. The 
slitless spectroscope, in the hands of various observers 
since 18Y0, has shown a continuous band of lio:ht with 
several smooth, bright rings upon it : the brightest and 
largest ring was green (corresponding to the 1,474 line), 
and there were three other fainter ones in the red, blue, 
and violet, corresponding to the three brightest lines of 
hydrogen. It is to be inferred, therefore, that the gas- 
eous matter of the corona forms a pretty regular atmos- 
phere around the sun, and that the structural elements, 
the rays, rifts, and streamers, are mainly due to mist or 
dust — at least they seem to give a continuous spectrum. 
With this agrees the fact, before mentioned, that the 
1,474 line is just as bright in the middle of one of the 



THE CORONA. 263 

dark rifts as in a bright streamer. In 18Y8 the slitless 
spectroscope, however, failed, in the hands of all the 
observers, to show any rings at all. This fact, taken 
with the lessened brightness of the corona on that occa- 
sion, seems to indicate that the gases of the coronal at- 
mosphere, at the time of a sun-spot minimum, are much 
diminished in extent and brilliance, while the streamers 
are comparatively unaffected. 

RAPID CHANGES IN THE CORONA. 

The question has been often raised, whether the 
appearance of the corona changes during an eclipse. 
Many drawings seem to show that this is the case ; they 
represent the corona at the beginning and end of the 
eclipse as much wider on that side of the sun less deeply 
covered by the moon — on the western edge, near the 
beginning of the eclipse, and on the eastern, near its end 
— while it is approximately symmetrical at the middle 
of totality ; and this circumstance was much relied upon 
for a time by those who maintained that the corona is, 
in the main, a phenomenon of the earth's atmosphere. 
Other drawings, however, of the same eclipses, show 
nothing of the kind, nor do the photographs, except in 
one or two instances, where a sufficient explanation is to 
be found in drifting clouds. On the other hand, pho- 
tographs taken at different moments during an eclipse, 
and at stations many hundred miles apart, agree so close- 
ly as to make it evident that the main features of the 
corona change only gradually, persisting, as a rule, for 
hours at least, and perhaps for days and weeks for aught 
we know. At the same time they do sometimes change 
perceptibly^ even in the course of twenty minutes, while 
the shadow is traveling between stations only a few 
hundred miles apart. Some have thought they saw 



264 THE SUN. 

rapid movements in the streamers, and have described 
them as waving and flickering ; one or two have even 
imagined that the corona "whirled like a catherine- 
wheel." Probably this is mere imagination, though 
the unsteadiness of the air might give a person unused 
to astronomical observation the idea of quivering mo- 
tion. The usual impression upon the mind is quite 
difl'erent — that of calm, serene stability. 

Combining the facts that have been ascertained, and 
speaking in the most general way, it would seem that 
the corona is mainly composed of filanaents which either 
emanate from the sun or are developed in his atmos- 
phere most abundantly at those portions of his surface 
about midway between the equator and the poles, those 
filaments which are emitted on either side of the zone 
having a tendency to lean toward the central ones. As 
a consequence, the corona tends toward the form of a 
four-rayed star, the points of which are inclined 45° to 
the sun's axis, and are made up of converging filaments, 
constituting the synclinal structure which Mr. Kanyard 
first clearly brought out. 

Obviously, however, this statement must be taken 
very loosely. Every eclipse presents striking excep- 
tions. There are always streamers tangential, curved, 
or inclined, which can be brought imder no such rule ; 
faint, far-reaching cones of light, like those which were 
seen in 1878 ; dark rifts, rounded masses of nebulosity, 
vortices, and a multitude of other peculiarities of struct- 
ure no more reducible to a formula than the shapes of 
flame or cloud. 

Opinion is very widely divided as to the nature 
and origin of the substances which compose the coronal 
structures. Very few now, we think, deny the pres- 
ence of an atmosphere of incandescent gases reaching 



THE CORONA. 265 

to an elevation of at least 300,000 miles, and this al- 
though there are enormous difficulties in harmonizing 
an atmosphere of such extent with the low pressure at 
the surface of the photosphere, indicated by the fineness 
of the Fraunhofer lines in the spectrum. But, as to 
the material of which the streamers are composed, and 
the nature of the forces which determine their form 
and position, their is no agreement. Some see in the 
corona simply flocks of meteors, and there can be no 
doubt that meteoric matter must abound in the sun's 
immediate neighborhood. But looking, for instance, at 
the pictures of the eclipse of 18Y1 and 1889 it appears 
evident that the details of that corona could not be ac- 
counted for in this way. It seems much more likely 
that the phenomena of comets' tails and the streamers of 
the aurora are phenomena of the same order, and though 
as yet the establishment of this relation would not 
amount to anything like an explanation of the corona, 
it would be a step toward it — a step by no means taken 
yet, however, it must be admitted ; nor is it easy to see 
at present how the problem is to be attacked. That the 
forces concerned reside in the sun himself is made prob- 
able by the usual approximate symmetry of the corona 
with reference to his axis, and the fact that the coronal 
streamers seem to originate most abundantly nearly in 
the sun-spot zones. 

But we must evidently wait awhile for the solution 
of the problems presented by the beautiful phenomenon. 
Possibly the time may come when some new contrivance 
may enable us to see and study the corona in ordinary 
daylight, as we now do the prominences. The spectro- 
scope, indeed, will not accomplish the purpose, since 
the rays and streamers of the corona give a continuous 
spectrum ; but it would be rash to say that no means 



266 THE SUN. 

will ever be found for bringing out the structures around 
the sun which are hidden by the glare of our atmos- 
phere. Unless something like this can be done, the 
progress of our knowledge must probably be very slow, 
for the corona is visible only about eight days in a cent- 
ury, in the aggregate, and then only over narrow stripes 
on the earth's surface, and but from one to five minutes 
at a time by any one observer."^ 

Within the past few years a number of very earnest 
attempts have been made in the direction indicated. 
Dr. Huggins was the first to move, and from 1883 for 
a number of years worked hard in the endeavor to ob- 
tain photographs of the corona in full sunshine. He 
succeeded very early in getting a number of plates show- 
ing around the sun certain faint, elusive halo-forms 
which certainly look very coronal. Plans were made, 
and were carried out, in 1884, for using a similar appa- 
ratus upon the Riiielberg in Switzerland, and after- 
ward at the Cape of Good Hope. Nothing has been 
obtained, however, much in advance of Dr. Huggins's 
own first results. But since September, 1883, until late 
in 1885, the air, as every one knows, was full of a fine 
haze, probably composed in the main of dust and vapor 
from Krakatoa, whicli greatly interfered with all such 
operations. 

About the same time that Dr. Huggins was photo- 
graphing in England, Professor Wright, of New Haven, 
Avas experimenting on the same subject in a different 
way. He reflected the sun's rays into a darkened room 

* This estimate is based upon the fact that total eclipses occur on the 
average about once in two years, that the shadow occupies (on the aver- 
age, again) some three hours in traversing the globe, and that the mean 
duration of totality is between two and three minutes, never by any pos- 
sibility reaching eight minutes, and very seldom six. 




THE CORONA. 267 

by a heliostat, cut out all but the blue and violet rays by 
a suitable absorbing-cell, and then formed an image of 
the sun and its surroundings upon a sensitive fluorescent 
screen, stopping out the sun's disk itself. He obtained 
on the screen, on more than one occasion, what he then 
believed and still believes to be a true image of the 
corona. But the aerial haze soon intervened to put an 
end to all such operations ; for of course it is evident 
that success, whether by photography or by fluorescence, 
is possible only under conditions of unexceptionable at- 
mospheric purit}^ 

Both Professor Wright and Dr. Huggins base their 
hopes upon the belief, which seems to be warranted by 
the spectrum-photographs obtained during the Egyptian 
eclipse of May, 1882, that the light of the corona aud of 
the upper regions of the sun's " atmosphere • ' (if one 
may so speak of what is not strictly an " atmosphere " 
at all) is peculiarly rich in violet and ultra-violet rays 
— that the corona is far more brilliant to the photo- 
graphic plate and to the fluorescent screen than to the 
eye. 

The reports from the eclipse of August 29, 1886, ob- 
served by English and American parties on the island 
of Grenada in the Southern West Indies were strongly 
unfavorable to the reality of the coronal appearances 
obtained by Huggins and Wright in their attempts to 
render the corona visible without an eclipse. Plates 
furnished by Mr. Huggins, and precisely similar to those 
which he has employed in his photographic experiments, 
were exposed by Captain Darwin during the totality (as 
well as before and after it), in an apparatus like Mr. 
Huggins's, with a time of exposure the same that he has 
been using, and were treated and developed in accord- 
ance w^ith his directions. The plates which were thus 



268 THE SUN. 

exposed during the totality show no corona at all^ the 
exposure time having proved insufficient to bring it out. 
Nor do the plates exposed during the partial phase show- 
any trace of the moon's outline beyond the sun's limb. 
Of course this makes it extremely probable that what 
looks like the corona upon plates exposed in the same 
way to the uneclipsed sun is merely a fallacious ghost, 
due, as his opponents have always claimed, to something 
in his apparatus or process, or else to the scattering of 
light in our atmosphere. It is true, as Mr. Common 
points out, that the result is not absolutely conclusive, 
because the air was by no means satisfactorily clear dur- 
ing the eclipse ; but it must be conceded, and Mr. Hug- 
gins himself admits it, that the probability is now heavi- 
ly against him. Captain Darwin obtained good pictures 
of the corona with ordinary plates exposed for a longer 
time in the usual apparatus. 

The later eclipses of 1889 and 1893 also bear in the 
same direction. 

More recently still. Professor Hale has made a new 
attempt wdth the spectro -heliograph, from Pike's Peak 
and the top of ^tna, as well as from his own observa- 
tory. He was in hopes that by the use of the double 
slit of the instrument, shutting out all but the " K light," 
which is especially strong in the spectrum of the corona, 
the effect of the aerial illumination might be relatively 
reduced to a great degree, because in the spectrum of 
the air light, " K light " is almost w-anting — in the air 
spectrum K is a black band. But he has had no better 
success than his predecessors. 

He is now about to try still another method, and 
endeavor to make the corona manifest itself by its heat- 
rays^ using for the purpose a bolometric apparatus very 
similar to that with which Professor Langley has beeni 






THE CORONA. 269 

making his remarkable investigation of the infra-red 
spectrum. 

Probably it must be admitted that at present the 
predominant opinion among astronomers and photog- 
raphers is against the practicability of reaching the 
corona without an eclipse, by any such methods ; still, 
to the writer at least, the case appears by no means ab- 
solutely hopeless, and success is certainly devoutly to be 
desired. 

We must not close the chapter without a few words 
as to the recent course and the present state of theoret- 
ical speculation respecting the corona. 

At the eclipse of 1883, observed on Caroline Island, 
in the Pacific Ocean, by French and American parties. 
Professor Hastings made observations for the purpose 
of testing a theory he had framed, that the outlying re- 
gions of the corona are merely a diffraction effect pro- 
duced by the edge of the moon ; the diffraction being 
not that due to the regular periodicity of light- vibra- 
tions, ordinarily discussed, but due to the probable con- 
tinually occurring discontinuity or change of phase in 
the vibrations. It seems probable, from a not perfectly 
complete investigation, that such discontinuity might 
scatter light far beyond the limits of ordinary diffrac- 
tion. He found during the eclipse, by an apparatus 
constructed expressly for the purpose, that the bright 
corona-line (1474: K) was always visible to a much 
greater distance from the sun on the side least deeply 
covered by the moon than on the other, as unquestion- 
ably ought to be the case if his theory were correct. 

But the same thing would result from the diffusion 
of light by the air ; and the French observers, and nearly 
all others who have discussed the matter, feel satisfied 
that this is the true explanation of what he saw. He 



270 THE SUN. 



1 



himself now, we understand, thinks it not impossible 
that a thin cloud may have passed over the sun just at 
the critical moment, and so have vitiated his observa- 
tion. 

The discussion which has followed his publication 
seems to have only strengthened the older view, that the 
corona is a true solar appendage, an intensely luminous 
though inconceivably attenuated cloud of gas, fog, and 
dust, surrounding the sun, formed and shaped by solar 
forces. 

The fact that comets, tliemselves mere airy nothings, 
have several times (the last instance was in 1882) passed 
absolutely through the corona without experiencing any 
sensible disturbance of path or structure, has, however, 
been always felt by many as an almost insuperable diffi- 
culty with this accepted theory, and more than anything 
else led Professor Hastings to propose his new hypothe- 
sis. But, on careful consideration, we shall find that 
our conceptions of the possible attenuation of shining 
matter near the sun will bear all the needed " stretch- 
ing" without involving any absurdity. Recalling the 
phenomena of the electrical discharge in Crookes's tubes, 
it is clear that a " cloud," with perhaps only a single 
molecule to the cubic foot (but thousands of miles in 
thickness), would answer every luminous condition of 
the phenomena. And all the rifts and streamers, and 
all the peculiar structure and curved details of form, cry 
out against the diffraction hypothesis. 

Professor Schaeberle, of the Lick Observatory, has 
proposed a very different hypothesis, which he calls a 
" mechanical " theory of the solar corona. As a basis 
he assumes that the eruptions from the sun's surface are 
most active and numerous in the spot-zones, and that 
the sun rotates upon an axis inclined 82f ° to the plane 



THE CORONA. 271 

of the earth's orbit. Then, in his own words, " the theo- 
retical corona is caused by light emitted and reflected 
from streams of matter ejected from the sun by forces 
which in general act along lines normal to the surface 
of the sun, these forces being most active near the cen- 
ter of each sun-spot zone." 

Many of the apparent variations in the type of the 
corona wall therefore depend upon the perspective un- 
der which these streams are seen, according to the time 
of year ; others upon the relative abundance and force 
of the streams on different portions of the solar surface 
according to the phase of the sun-spot period at the 
time ; and others yet, the curved rays especially, are 
caused by optical illusions due to the apparent crossing 
and interlacing of streams that lie in different planes. 

If we assume, as Mr. Schaeberle does, that the ejected 
material which forms the streamers leaves the sun with 
a velocity which may be as great as nearly four hundred 
miles a second, and returns with the same velocity after 
having traveled as far away as the orbits of Jupiter and 
Saturn, it appears that the neighborhood of the sun 
ought to be filled with a diffuse shower of swiftly de- 
scending dust, met and penetrated by the more definite 
and concentrated ascending jets. In the interplay and 
confiict of the rising and falling materials Mr. Schae- 
berle thinks he finds the explanation of the periodicity 
of the sun-spots, while he accounts for the existence of 
the sun-spot zones by the manner in which the heated 
gases, rising from the center of the cooling globe of the 
sun, would reach the surface and produce in the photo- 
sphere zones of greater and lesser thickness — belts of 
surface strength and weakness. For a more detailed 
account of this theory, which has already gained pretty 
wide acceptance, we must refer the reader to its pro- 



272 THE SUN. 

poser's original papers in the Lick Observatory report 
upon the eclipse of December, 1889, and in Yol. xiii 
of " Astronomy and Astro-Physics," April, 1894. It 
gives, however, no intelligible account of the apparent 
spectroscopic differences between the chromosphere and 
corona, and the magnetic forces acting at the sun do not 
appear in it at all. 

What may be considered as the principal rival theory 
regards the corona as very like a permanent " aurora " 
around the sun, the position and direction of its stream- 
ers being determined by the sun's magnetic field of 
force, in the same way that the terrestrial lines of 
magnetic force direct the beams of our own " aurora 
borealis." r 

Professor Bigelow, now of Columbia College^ has 
recently investigated the subject mathematically, and 
apparently with great success so far as concerns the dis- 
tribution, curvature, and general appearance of the 
streams and filaments which compose the corona. He 
finds that in the sun, as in the earth, the magnetic axis 
does not coincide with the axis of rotation, the sun's 
north magnetic pole being distant 4J°, and the southern 
9^°, from the corresponding pole of rotation. He finds 
that in the corona of the eclipse of 1878 (the photo- 
graphs of which he was able to obtain for measurement) 
the force which directs the streamers appears to be re- 
pulsive, and that the bases of the individual streamers, 
not very numerous but of enormous dimensions, are 
mainly grouped in a zone some 10° wide, with the 
greatest density about 34° from the coronal poles, while 
their visible upper extremities are located about 500,- 
000 miles vertically above the sun-spot belts. He adds : 
"At this place the incandescence of the material parti- 
cles apparently ceases, and if condensation sets in there 



THE COROXA. 273 

would exist the conditions required for the precipitation 
of cool masses, whose fall upon the surface of the sun is 
generally supposed to produce the spots." He considers 
that the " structureless equatorial wing is no doubt a 
floating mass of matter cooling in the process of prepa- 
ration for precipitation." For further details we must 
refer the reader to Professor Bigelow's papers in the 
"American Journal of Science" from 1891 to 1894. 

As to the origin of the repulsive force which causes 
the projection of the streamers from the polar regions, 
Professor Bigelow does not commit himself, though 
evidently having in view the idea that it may be " elec- 
trical " in the broad sense of the word. 

We must not close the chapter without at least a 
reference to the beautiful experiments of Dr. Pupin, of 
New York. Under certain conditions he obtains mag- 
nificent "coronoidal discharges" from the surface of 
a brass ball inclosed within a large glass globe, from 
which the air has been more or less perfectly exhausted. 
The photographs are certainly very suggestive, and seem 
to show that if there are violent electric disturbances 
upon the sun — " solar thunderstorms," so to speak — 
they might inductively produce coronal streamers. The 
possibility, however, of such thunderstorms at solar tem- 
peratures is extremely doubtful at present, and the sub- 
ject must be left for further development. 



19 



CHAPTER YIIL 

THE SUN'S LIGHT AND HEAT. 

Sunlight expressed in Candle-Power. — Method of Measurement. — Bright- 
ness of the Sun's Surface. — Langley's Experiment. — Diminution 
of Brightness at Edge of the Sun's Disk. — Hastings's View as to 
Nature of the Absorbing Envelope. — Total Amount of Absorption by 
Sun's Atmosphere. — Thermal, Luminous, and Actinic Rays : their 
Fundamental Identity and Differences. — Measurement of the Sun's 
Radiation. — Herschel's Method. — Expressions for the Amount of 
Sun's Heat. — Pouillet's Pyrheliometer. — Crova's. — YioUe's Actinom- 
eter. — Langley's Researches. — Absorption of Heat by Earth's Atmos- 
phere ; by the Sun's. — Question as to Differences of Temperature on 
Different Portions of Sun's Disk. — Question as to Variation of Sun's 
Radiation with Sun-Spot Period. — The Sun's Temperature — Actual 
— Effective. — Views of Secchi, Ericsson, Pouillet, Vicaire, Rosetti, 
Le Chatelier, and Wilson and Gray. — Scheiner's Spectroscopic Evi- 
dence. — Evidence from the Burning-Glass. — Langley's Experiment 
with the Bessemer " Converter." — Permanency of Solar Heat for last 
Two Thousand Years. — Meteoric Theory of Sun's Heat. — Helmholtz's 
Contraction Theory. — Possible Past and Future Duration of the Sun's 
Supply of Heat. — Siemens's Untenable Theory. 

Sunlight is the intensest radiance at present known. 
It far exceeds the brightness of the calcium-light, and 
is not rivaled even by the most powerful electric arc. 
Either of these lights interposed between the eye and 
the surface of the sun appears as a black spot upon the 
disk. 

We can measure with some accuracy the total quan- 



THE SUN'S LIGHT AND HEAT. 2Y5 

tity of sunlight, and state the amount in " candle-pow- 
er" ; the figure which expresses the result is, however, so 
enormous that it fails to convey much of an idea to the 
mind— it is 1,575,000,000,000,000,000,000,000,000 :— fif- 
teen hundred and seventy-five billions of billions, enu- 
merated in the English manner, which requires a mil- 
lion million to make a billion ; or one octillion five hun- 
dred and seventy-five septillion, if we prefer the French 
enumeration. 

The '' candle-power," which is the unit of light gen- 
erally employed in photometry,^ is the amount of light 
given by a sperm-candle weighing one sixth of a pound, 
and burning a hundred and twenty grains an hour. 
An ordinary gas-burner, consuming five feet of gas 
hourly, gives, if the gas is of standard quality, from 
twelve to sixteen times as much light. The total light 
of the sun is therefore about equivalent to one hundred 
oillion billion of such gas-jets. 

This statement rests mainly upon the measurements 
made by Bouguer in 1725, and WoUaston in 1799 ; since 
then, however, confirmed by others. They found that 
the sun in the zenith would illuminate a white surface 
about sixty thousand times as intensely as a standard 
candle at the distance of one metre. Allowing for ab- 
sorption of light in our atmosphere, the figure would 
rise to about seventy thousand. As the distance of 
the sun is ninety-three million miles, or very nearly a 
hundred and fifty million kilometres, it follows that, 
if we multiply 70,000 by the square of 150,000,000,000 
(reducing kilometres to metres), the product will express 

- * The photometric unit proposed by the Paris International Congress 
in 1890 is one twentieth of the light emitted by a square centimetre of 
molten platinum just solidifying. It is called the " decimal candle," and 
is about one per cent, smaller than the old unit. 



276 



THE SUN. 



the number of candles which, at the sun's distance, 
would give a light equal to that of the sun. The num- 
ber comes out as stated above, though it is undoubt- 
edly uncertain by a considerable percentage. It de- 
pends upon old observations, which ought to be re- 
peated ; observations, also, which are difficult and 
never very satisfactory because of the vagueness of 
the unit, the extreme difference between the intensity 
of the lights compared, and, what is still more trouble- 
some, the difference between the color of the sun- 
light and of candle-light. 



Fig. 94. 



.''''r\l 



H 




Method of measuring the Intensity of SuNLronT. 



The method of making such a comparison is illus- 
trated by Fig. 94. A mirror, J/, throws the rays of the 
sun into a darkened room upon a small lens, the diam- 
eter of which is accurately known. This lens brings 



I THE SUN'S LIGHT AND HEAT. 277 

the rays to a focus at i^, after passing which point they 
diverge and fall upon a white screen, a?, at a consider- 
able distance. Neglecting for the present the loss of 
light by reflection at the surface of the mirror and 
transmission through the lens, we may say that the 
illumination of the screen is as many times less than 
that of full sunlight as the area of the lens Z is less than 
that of the whole disk of light upon the screen. If, for 
instance, the lens is one fourth of an inch in diameter, 
and the circle of light on the screen is ten feet across, 
then the light on the screen would be 230,400 times 
fainter than sunlight. If we allow for the loss by re- 
flection and in the lens, the ratio would probably not 
be far from 300,000 to 1. Of course, these two correc- 
tions must be (and can be) accurately determined by 
special observations for the purpose. Having got thus 
far, there are various methods of proceeding. The 
simplest, and by no means the least accurate, is to place 
a small rod, like a pencil, near the screen, so that its 
shadow will be cast by the sunlight at a : the candle of 
comparison, (7, is then moved back and forth until a 
position is found at which the shadow cast by its flame 
at i is equally strong with the other shadow. Then the 
relative amounts of illumination on the screen produced 
by the sun and by the candle will be as tlie squares of 
the lines a F and h C. There are other methods ad- 
mitting of still greater precision, but all embarrassed 
(as this is) by the difference of color between sun- and 
candle-light. The most uncertain part of the operation 
lies, however, in the corrections for loss of light in the 
atmosphere, at the mirror, and in the lens. 

Thus far we have considered only the total light 
emitted by the sun. The question of the intrinsic 
brightness of his surface is a different though connected 



278 THE SUN. 

one, depending for its solution upon the same observa- 
tions, combined with a determination of the light-radi- 
ating areas in the different cases. Since a candle-flame 
at the distance of one metre looks considerably larger 
than the disk of the sun, it is evident that it must be a 
good deal more than seventy thousand times less brill- 
iant. In fact, it would have to be at a distance of 
about 1*65 metres to cover the same area of the sky as 
the sun does, and therefore the solar surface must ex^ 
ceed by a hundred and ninety thousand times the aver- 
age brightness of the candle-flame. 

In the calcium-light the luminous point is both 
much more brilliant and much smaller than a candle^ 
flame, so that the discrepancy is considerably less. Ac- 
cording to certain experiments by Foucault and Fizeau 
in 1844, the solar surface was found to be a hundred 
and forty-six times more brilliant than the incandescent 
lime. At the same time they experimented upon the 
electric arc, and found the brightest part of this to be 
only about four times fainter than the sun. Their ex- 
periments were, however, conducted by exposure of a 
Daguerreotype-plate to the rays to be compared, and 
there is room for considerable doubt as to their accuracy. 
Later experiments have sliowed in some cases a rather 
higher intensity for the brightness of the positive car- 
bon of the electric arc (which is always much more brill- 
iant than the negative). It is asserted in a few instances 
to have reached a brilliance fully half as great as that 
of the solar surface ; but the evidence is not entirely 
satisfactory, the comparisons being only indirect. The 
magnificent lights produced by the dynamo -electric 
machines of the present day differ from that employed 
by Foucault and Fizeau, not so much in intensity as in 
quantity. The illuminating surfaces are larger, and the 



THE SUN'S LIGHT AND HEAT. 279 

extent of the arc much greater, but the brightness of 
the luminous points concerned seems to remain about 
tlie same, and probably depends mainly upon the phys- 
ical characteristics of the carbon, which are essentially 
the same in all cases. 

One of the most interesting observations upon the 
brightness of the sun is that of Professor Langley, who, 
a few years ago (in 1878), made a careful comparison 
between the solar radiation and that from the blinding 
surface of the molten metal in a Bessemer " converter." 
The brilliance of this metal is so great that the dazzling 
stream of melted iron, which, at one stage of the pro- 
ceedings, is poured in to mix with the metal already in 
the crucible, " is deep brown by comparison, presenting 
a contrast like that of dark coffee poured into a white 
cup." The comparison was so conducted that, inten- 
tionally, every advantage was given to the metal in 
comparison with the sunlight, no allowances being made 
for the losses encountered by the latter during its pas- 
sage through the smoky air of Pittsburg to the reflector 
which threw its rays into the photometric apparatus. 
And yet, in spite of all this disadvantage, the sunlight 
came out Jive thousand three hundred times brighter 
than the dazzling radiance of the incandescent metal. 

Thus far we have spoken of the sun as a whole, but, 
as has been said before, there is a marked diminution 
of the light at the edges of the disk ; so marked, indeed, 
that it is exceedingly surprising that any person should 
ever have questioned the fact, as some — Lambert, for 
instance — have done. Arago came very near it, for he 
set the difference at only -^^ — so little as to be hardly 
perceptible. An image of the sun a foot in diameter, 
formed by a small telescope of two inches' aperture, 
upon a white paper screen, shows the fact, however, in 



280 THE SUN. 

an entirely unquestionable manner. Many measure- 
ments have been made for the purpose of comparing Ai 
the brightness of different parts of the disk. Professors 
Pickering and Langley, in this country, and Vogel, in 
Germany, are among the most recent and reliable inves- 
tigators of the subject. Professor Pickering eflfected 
his measurements by forming, with a small telescope, 
an image of the sun, about sixteen inches across, upon a 
white screen perforated with an orifice three fourths of 
an inch in diameter. The telescope was placed horizon- 
tally, and the light directed upon it by a mirror, much as 
in the preceding figure, except that the mirror was moved 
by clock-work, so as to keep the image constantly in one 
place. After the rays passed the orifice in the screen 
they were received upon the disk of a Bunsen pho- 
tometer, and the light compared with that of a standard 
candle, in the ordinary way, and thus the ratio was 
found between the brilliance of the center of the disk 
and that of other parts. Pickering makes the ratio 
between the intensity of the light from the edge and 
center to be thirty-seven per cent. 

Vogel, in 1877, proceeded still more elaborately. 
His instrument, called a spectral photometer, enabled 
him to compare with great accuracy, and directly, the 
brightness of the rays of different colors proceeding from 
different parts of the sun — the red rays by themselves, 
and the same with the yellow, green, blue, and violet. 
The following table contains an abridgment of his re- 
sults. In the first column, headed D, is given the distance 
of the point from the sun's center in percentage of the 
sun's radius. The other columns give the ratio between 
the light of the given color at the center of the disk 
and at the point in question, expressed also as a per- 
centage. Thus, at the very edge of the disk, at a dis- 



TOE SUN'S LIGHT AND HEAT. 



2S1 



tance of one hundred per cent, of the sun's radius from 
its center, the violet light has an intensity of only thir- 
teen per cent, of its intensity at the center, and the red 
thirty per cent, of its central intensity : 



D. 


Violet, 


Blue. 


Green, 


Yellow, 


Eed, 


Pickering. 


A 408. 


A 470. 


A 512. 


A5b9. 


662. 


general light. 





100 


100 


300 


100 


100 


100 


10 


99-6 


99-7 


99-7 


99-8 


99-9 


98-8 


20 


98-5 


98-8 


98-7 


99-2 


99-5 


.... 


30 


96-3 


97-2 


96-9 


98-2 


98-9 




40 


93-4 


941 


94-3 


96-7 


980 


94 


50 


88-7 


91-3 


90-7 


94-5 


96-7 


91-3 


60 


82-4 


87-0 


85-2 


909 


94-8 


87-0 


- 10 


74-4 


80-8 


80-0 


84-5 


91-0 


.... 


75 


69-4 


76-7 


75-9 


8oa 


88-1 


78-8' 


80 


63-7 


71-7 


70-9 


74-6 


84-3 




85 


56-7 


65-5 


64-7 


67-7 


79-0 


69-2 


90 


47-7 


57-6 


566 


59-0 


71-0 


.... 


95 


34-7 


45-6 


44-0 


46-0 


58-0 


55-4 


100 


130 


16-0 


18-0 


25-0 


30-0 


37-4 



We have added, in a last column, some of the results 
of Professor Pickering, which, it will be seen, for the 
most part are in quite satisfactory accordance with those 
of Yogel. 

One thing is obvious from Vogel's table, namely, 
that the color of the light must be different at the edge 
of the disk from what it is in the center, since more of 
tho violet light than of the red is lost at the limb. 

Professor Langley, in 1875, in attempting to meas- 
ure directly the relative brightness of points near the 
center and limb by bringing, in a very ingenious man- 
ner, the light from the two points to confront each 
other on a Bunsen photometer-disk, found this to be a 
very noticeable fact — the edge is of a sort of chocolate- 
brown and the center quite bluish, if we take ordinary 
sunlight as the standard of whiteness. The difference 
of tint was suflSciently decided to make the measures 



282 THE SUN. 

very difficult. We have never seen in print the results 
of this work of his, and do not know whether thej have 
yet been published. Vogel's work, however, from the 
greater completeness of its analysis in respect to the 
different colors, must take the precedence of everything 
hitherto done in this line. 

Tlie cause of this enfeeblement of the light near the 
limb of the sun is, of course, the absorption of a portion 
of the rays by the solar atmosphere."^ It becomes, 
therefore, an interesting subject of inquiry, how much 
of the sunlight is thus absorbed — how much brighter 
the sun would shine if suddenly stripped of its gaseous 
envelopes ? 

Unfortunately, the question does not, in the present 
state of science, admit of a certain and definite answer. 
By making certain assumptions as to the constitution 
of the luminous surface and the character of the atmos- 
phere we may, it is true, deduce mathematical formulae 

* It has generally been considered that this absorbing envelope must 
b3 gaseous, and it lias usually been idci^tified with the so-called reversing 
layer. Professor Hastings, of New Haven, has, however, proposed a some- 
what different theory, viz., that the absorption is produced by matter 
in a pulverulent condition, at a lower temperature than the photospheric 
clouds, and disseminated through the lower portions of the sun's true 
atmosphere. He urges with force that the absorption of gases, at such a 
temperature, must be selective^ producing bands and lines in the spectrum, 
while the absorption with which we have to do in this case is general^ 
simply weakening all the rays pretty much alike, though of course affect- 
ing those of short-wave length more than those of long, as previously 
pointed out by Langley. The substance concerned, he fays, must be one 
which condenses and precipitates at a temperature higher than that of 
the photosphere, so that its vapor would not be present to any appreci- 
able extent in the photosphere and reversing layer, and its lines would 
not be found in the solar spectrum. He suggests that the substance is 
very probably carbon, the lines of which, at the time he wrote, had not 
been detected, though since discovered by Lockyer and Rowland. It is 
difficult at present to determine its real identity. 



THE SUN'S LIGHT AND HEAT. 283 

(of a rather complicated character) which will represent 
the observed facts on those assumptions. 

Laplace, for instance, assumed that each point npon 
the luminous surface of the sun radiated equally in all 
directions, and that its atmosphere was homogeneous 
throughout — knowing, of course, that it could not be 
homogeneous, but not knowing w^hat laws of density 
and temperature would apply in the case, and therefore 
not being able to supply a more correct hypothesis. 
On these assumptions, and taking as a basis of calcula- 
tion the observations of Bouguer, which in the main 
agree with the more modern ones, he found that the 
solar atmosphere must absorb about eleven twelfths of 
the whole light ; in other words, that the sun, without 
its atmosphere, would be about twelve times as bright 
as we see it now. Secchi has also adopted his conclu- 
sion. 

His first assumption, however, is probably very far 
from true. So far as we know, no luminous surface 
behaves as he supposes, but generally the radiations at 
an oblique angle are vastly less powerful than those 
perpendicular to the surface. According to Laplace's 
assumption, the sun, without its atmosphere, would be 
much brighter at the edge than at the center. Now, an 
incandescent sphere of metal, or an illuminated globe of 
white glass (like the shade of a student-lamp), appears 
sensibly of equal brightness all over, the foreshortening 
of each square inch of surface inclined to the line of 
sight just compensating for its diminished radiation. 
Assuming this law" of radiation for the solar surface, 
and still keeping the hypothesis of a homogeneous at- 
mosphere. Professor Pickering shows that the observed 
darkening from the center to the edge of the sun's disk, 
indicated by his measures, would be accounted for pretty 



284 THE SUN. 

accurately by supposing this atmosphere to have a height 
approximately equal to the sun's radius, and of such 
absorbent power as to reduce the light by about seventy- 
four per cent, at the center of the disk, leaving twenty- 
six per cent, to pass. From this it is possible to show 
that the whole light, if there were no solar atmosphere, 
would be about four and two thirds times as great as 
now — always, be it remembered, accepting the assump- 
tions. 

Vogel, assuming the same fundamental law of radia- 
tion, finds from his observations that the removal of the 
solar atmosphere would increase the brightness of its 
red rays about 1*49 times, and of the violet 3-01. The 
difference between this result and that of Pickering is 
larger than would be expected from the general near 
accordance of the observations, but is probably princi- 
pally due to the fact that Yogel employs a formula of 
Laplace's which implicitly assumes the solar atmosphere 
to be very thin as compared with the size of the sun 
itself, while Pickering's method of calculation accepts 
no such limitation. There is an important difference 
also between the observations of the two investigators 
near the edge of the disk : Vogel's observations show a 
much more rapid degradation of the light just there, 
and so indicate an atmosphere much denser, but of less 
elevation than Pickering's. 

It is evident, however, that for the present we must 
content ourselves with the rather vague statement that 
the removal of the sun's atmosphere would multiply its 
brio:htness several times. It is almost certain that the 
amount of light received by the earth would be doubled ; 
it is hardly likely that it would be quintupled. More- 
over, its color would be materially changed, and its tint, 
as pointed out by Langley, would be more hlue than 



THE SUN'S LIGHT AND HEAT. 285 

now. The solar atmosphere reddens the light trans- 
mitted through it, in just the same way that our terres- 
trial atmosphere does at sunset, but to a less degree. 

Thus far we have confined ourselves to those radia- 
tions which affect the sense of vision. But these rays 
do more : if received upon a dark surface they are, as 
we say, '^ absorbed," and the absorbing body becomes 
warmer. Nothing in science is now much more certain 
than that these luminous radiations consist of pulses of 
inconceivable (but measurable) frequency, which are 
communicated through intervening space ; pulses which 
are capable not merely of affecting the visual nerves of 
sentient beings, but of producing also many other effects, 
physical, thermal, or chemical, according to the surface 
which receives them. The human eye, however, is very 
circumscribed in its range of perception, taking cogni- 
zance only of such vibrations as do not exceed or fall 
short of certain limits of frequency — the slowest oscilla- 
tions it recognizes being those of the extreme red, which 
number about three hundred and ninety millions of 
millions of vibrations in a second ; while the most rapid, 
those of the extreme violet, are nearly twice as frequent, 
making seven hundred and seventy millions of millions 
in the same time. The rays -emitted by the sun are not, 
however, so limited ; but the visual vibrations are accom- 
panied by others both manj^ times more slow and more 
rapid. There has been a prevailing idea for many years, 
founded upon Brewster's fallacious experiments, that 
thermal, luminous, and chemical rays are fundamentally 
different, though coexistent in the sun's beams. This 
is erroneous. It is true, indeed, that rays whose vibra- 
tions are too slow to be seen produce powerful heating 
effects, and that those which are invisible because they 
are too rapid have a strong influence in determining 



286 THE SUN. 

certain chemical and physical reactions ; but it is also 
true that the visible rays are capable of producing the 
same effects to a greater or less degree, and there is 
some reason for thinking that certain animals can see 
by rays to which the human retina is insensible. There 
is absolutely no philosophical basis for distinction be- 
tween the visible and invisible radiations of the sun, 
except in the one point of vibration-frequency — their 
pitchy to use the analogy of sound. The expressions 
thermal, luminous, and chemical rays are apt to be mis- 
leading. All the waves of solar radiation are carriers of 
energy, and when intercepted do work, producing heat, 
or vision, or chemical action, according to circumstances. 

If the amount of solar light is enormous, as com- 
pared with terrestrial standards, the same thing is still 
more true of the solar heat, which admits of somewhat 
more accurate measurement, since we are no longer 
dependent on a unit so unsatisfactory as the " candle- 
power," and can substitute thermometers and balances 
for the human eye. 

It is possible to intercept a beam of sunshine of 
known dimensions, and make it give up its radiant 
energy to a weighed mass of water or other substance, 
to measure accurately the rise of temperature produced 
in a given time, and from these data to calculate the 
whole amount of heat given off by the sun in a minute 
or a day. 

Pouillet and Sir John Herschel seem to have been 
the first fairly to grasp the nature of the problem, and 
to investigate the subject in a rational manner. 

Herschel's experiments were made in 1838 at the 
Cape of Good Hope, where he was then engaged in his 
astronomical work. He proceeded in this way: A 
small tin vessel, containing about half a pint of water3 



THE SUN'S LIGHT AND HEAT. 2S7 

carefully weighed, was placed on a light wooden sup- 
port, touching it at only three points. This was put 
inside of a considerably larger cylinder, also of tinned 
iron, this outer cylinder having a double cover with a 
hole in it, the cover large enough to shade the sides of 
the vessel, and the hole a little less than three inches in 
diameter. A delicate thermometer was immersed in 
the water, with a sort of dasher of mica for the purpose 
of stirring it and keeping the temperature uniform 
throughout the mass. The apparatus w^as so placed and 
adjusted that the whole of the light and heat passing 
through the hole in the cover would fall upon the sur- 
face of the water, the sun at that time (December 31st) 
being within 12° of the zenith at noon. 

This apparatus was placed in the sunshine and al- 
lowed to stand for ten minutes, shaded by an umbrella, 
and the slight rise in the temperature of the water was 
noted. Then the umbrella was removed and the solar 
rays were allowed to fall upon the water for the same 
length of time, and the much larger rise of temperature 
was noted. Finally, the apparatus was again shaded, 
and the change for ten minutes again observed. The 
mean between the effects in the iirst and last ten-minute 
intervals could be taken as the measure of the influence 
of other causes besides the sun, and deducting this from 
the rise during the ten minutes' insolation, we have the 
effect of the simple sunshine. 

Herschel's figures for his first experiment run as 
follows : 

Rise of temperature in first ten minutes 0'°25 

" " '' " second ten minutes (sun) 3-°90 

" " " " third ten minutes °10 

The mean of the first and third is 0'°17, and this de- 
ducted from the second gives 3*°73 as the rise of tern- 



288 THE SUN. 

perature produced by a sunbeam three inches in diam- 
eter, absorbed by a mass of matter equivalent to 4,638 
grains of water (we do not indicate the minutiae of the 
process by which the weight of the tin vessel, ther- 
mometer, stirrer, etc., are allowed for). Nothing more 
is now necessary to enable us to compute just how much 
heat is received by the earth in a day or a year, except, 
indeed, the determination of the very troublesome and 
somewhat uncertain correction for the absorption of 
heat by the earth's atmosphere— a correction deduced 
by means of observations made at varying heights of 
the sun above the horizon. 

Herschel preferred to express his results in terms 
of melting ice, and put it in this way : the amount of 
heat received on the earth's surface, with the sun in 
the zenith, would melt an inch thickness of ice in two 
hours and thirteen minutes nearly. 

Since there is every reason to believe that the sun's 
radiation is equal in all directions, it follows that, if the 
sun were surrounded by a great shell of ice, one inch 
thick and a hundred and eighty-six million miles in 
diameter, its rays would just melt the whole in the 
same time. If, now, we suppose this shell to shrink in 
diameter, retaining, however, the same quantity of ice 
by increasing its thickness, it would still be melted in 
the same time. Let the shrinkage continue until the 
inner surface touches the photosphere, and, allowing 
for atmospheric absorption, the ice -envelope would 
become more than a mile thick, through which the 
solar fire would still thaw out its way in the same time 
— at the rate, according to Herschel's determinations, 
of more than forty feet a minute. Herschel continues 
that, if this ice were formed into a rod 45-3 miles in 
diameter, and darted toward the sun with the velocity 



THE SUX^S LIGHT AND HEAT. 289 

of liglit, its advancing point would be melted off as fast 
as it approached, if by any means the whole of the solar 
rays could be concentrated on the head. Or, to put it 
differently, if we could build up a solid column of ice 
from the earth to the sun, nearly two miles and a half 
in diameter, spanning the inconceivable abyss of ninety- 
three million miles, and if then the sun should concen- 
trate his power upon it, it would dissolve and melt, not 
in an hour, nor a minute, but in a single second : one 
swing of the pendulum, and it would be water ; seven 
more, and it would be dissipated in vapor. 

In formulating this last statement we have, however, 
employed, not Herschel's figures, but those resulting 
from later observations, which increase the solar radia- 
tion almost fifty per cent., making the thickness of the 
ice-crust which the sun would melt off of his own sur- 
face in a minute to be much nearer sixty feet than forty. 

To put it a little more technically, expressing it in 
terms of the modern scientific units, the sun's radiation 
amounts to more than 1,200,000 calories per minute for 
each square metre of his surface, the calori/^^ or heat- 
unit, being the quantity of heat which will raise the 
temperature of a kilogramme of water one degree cen- 
tigrade. 

An easy calculation shows that, to produce this 
amount of heat by combustion would require the hourly 
burning of a layer of anthracite coal more than nineteen 
feet (five metres) thick over the entire surface of the 
sun — nine tenths of a ton per hour on each square foot 
of surface — at least nine times as much as the consump- 

* This is the engineers' " calory." For many scientific purposes the 
*''' small calory^'''' a thousand times less, is more conveniently used — viz., 
the amount of heat which will raise the temperature of one gramme of 
v.atcr one centigrade degree. 
20^ 



290 THE SUN. 

tion of the most powerful blast-furnace known to art.! 
It is equivalent to a continuous evolution of about twelve! 
thousand horse-power on every square foot of the sun's! 
whole area. As Sir William Thomson (now Lord Kel- 
vin) has shown, the sun, if it were composed of solid! 
coal, and produced its heat by combustion, would burn 
out in less than five thousand years. 

Of this enormous outflow of heat the earth of course 
intercepts only a small portion, about ■2,-2-oo",'o^o~o,oto- 
But even this minute fraction is enough to melt yearly, 
at the earth's equator, a layer of ice more than one hun- 
dred and thirty-two feet thick. If we choose to express 
it in terms of " power," we find that this is equivalent, 
for each square foot of surface, to more than seventy- 
two tons raised to the height of a mile ; and, taking 
the whole surface of the earth, the average energy re- 
ceived from the sun is over sixty mile-tons yearly, or 
one horse-power continuously acting, to every twentj^- 
five square feet of the earth's surface. Most of this, 
of course, is expended merely in maintaining the earth's 
temperature ; but a small portion, perhaps j^V'o ^^ ^^ 
whole, as estimated by Helmholtz, is stored away by 
animals and vegetables, and constitutes an abundant 
revenue of power for the whole human race.^ 

* Several experimenters have contrived machines for the purpose of 
utilizing the solar heat as a source of mechanical energy, among whom 
Ericsson and Mouchot have been most successful. M. Pifre describes 
some results from a machine of Mouchot^s construction, claiming to have 
utilized more than seventy per cent, of the heat which falls on the mirrors 
of the instrument — something over twelve calories to a square metre. We 
do not mean, of course, that this percentage of the total solar energy ap- 
peared as mechanical power in the engine^ but only in its boiler. The 
machine had a mirror-surface of nearly a hundred square feet, and gave 
not quite a horse-power. Ericsson's engine, exhibited for several years 
in the American Institute Fairs in New York, about 1886, was still more 
efficient and powerful, driving a two and a half horse-power engine vigor- 



THE SUN'S LIGHT AND HEAT. 



291 



Fig. 95. 



If we inquire what becomes of that principal portion 
of the solar heat which misses the planet^ and passes off 
into space, no certain answer can be given. Remem- 
bering, however, that space is full of isolated particles 
of matter (which we encounter 
from time to time as shooting- 
stars), we can see that nearer or 
more remotely in its course each 
solar ray is sure to reach a rest- 
ing-place. It has been suggested 
that the sun sends heat only to- 
ward its planets ; that the action 
of radiant heat, like that of gravi- 
tation, is only hetween masses. 
But scientific investigation so far 
fails to prove it. The energy 
radiated from a heated globe is 
found to be alike in all direc- 
tions, and wholly independent of 
the bodies which receive it, nor 
is there the slightest reason to 
suppose the sun any way differ- 
ent in this respect from every 
other incandescent mass. 

Pouillet's experiments were 
made about the same time as Herschel's, but with a dif- 
ferent apparatus, though based on the same principles. 
He named his instrument the pyrheliometer, or " meas- 
urer of solar fire." Fig. 95 represents it. The little 
snuffbox-like vessel, a^ J, of silver-plated copper, black- 
ened on the upper surface, contains a weighed quantity 

ously. It is quite likely that such machines will prove practically useful 
in countries where sunshine can be depended on at certain seasons, as in 
Egypt and California. 




292 



THE SUN. 



of water, and a tliermometer is immersed in it, the mer- 
cury in its stem being visible at d. The disk, ^, e^ makei 
it easy to point the instrument squarely to the sun, bj 
directing it so that the shadow of a falls concentrically 
upon this disk. The button at the lower end is for the 
purpose of agitating the water in the vessel a, a, by sim- 
ply turning the whole thing on its axis, in the collar <?, c. 
The instrument is much more convenient than HerscheFs 
apparatus, but hardly as accurate, except under very care- 
ful manipulation and protection from currents of air. 



Fig. 96. 




Crova has modified it by filling the upper vessel 
with mercury. For relative measurements, as, for in- 
stance, a comparison of the amounts of heat received 
from the sun at diflferent hours, Crova employs a slight-J 
ly different instrument, represented in Fig. 96. 

An exceedingly sensitive alcohol thermometer, shown 
separately at T, with a large bulb carefully blackenedJ 



» 



THE SUN'S LIGHT AXD HEAT. 292 



is inclosed in a double-walled sphere, B^ nickel-plated 
on the outside. An opening in the walls of the sphere, 
carefully aligned with a similar opening in a double 
screen, E^ allows a beam of light to fall upon the ther- 
mometer-bulb, the beam being about two thirds the 
diameter of the bulb. The thermometer is constructed 
with a supplementary reservoir, r, at the lower end, bj^ 
means of which the end of the indicating column can 
be made to fall near the middle of the scale at any tem- 
perature, the object being to measure only changes of 
temperature, not absolute temperatures. The bulb and 
tube are so proportioned that a degree on the scale is 
nearly half an inch long, thus permitting great accuracy 
of reading. In order, however, to determine just how 
much heat is required to raise the thermometer of this 
instrument 1°, it is necessary to compare it with one of 
the standard instruments, by exposing it to the sun at 
the same time. 

This method of procedure, by which we determine 
the rate at which a sunbeam of given dimensions com- 
municates heat to a measured mass of matter, is known 
as the dynamic method. It is somewhat inconvenient 
in requiring considerable time and a number of readings. 

There is a different process for deducing the same 
results, which has been employed by Waterston, Erics- 
son, Secchi, Violle, and others, and may be called the 
statical method. It consists essentially in observing 
how much the sun will raise the temperature of a body 
exposed to its rays above that of the inclosure in which 
it is placed, this inclosure being kept at a fixed and 
known temperature by the circulation of water, or some 
such means. Instruments based on this principle are 
called actinmneters. Of these, probably the most com- 
plete in its arrangements is that of YioUe, described in 



294 



THE SUN. 



his paper upon the mean temperature of the sun's sur- 
face, published in the " Annales de Chimie/' in 1877. 
We give a diagram of the instrument. It consists of 
two concentric spheres of thin metal, the outer twenty- 
three centimetres in diameter, the inner fifteen centi- 
metres. The outer is polished on the outside ; the 



Fig. 97. 




a 



a 



ViOLLE S ACTINOMETE-:. 



loner is blackened on the inside. The space between 
the two spheres is filled with water, which is kept at a 
uniform temperature either by mixing snow or ice with 
it, or else by a current circulated through it by means 
of tlie stopcocks t^ t. A sensitive thermometer, T^ has 
its blackened bulb placed in the center of the inner 



( 



THE SUN'S LIGHT AND HEAT. 295 



sphere, the stem reaching outside through a tubuhire 
provided for the purpose. Two opposite openings, 
shown in the figure, allow a beam of sunlight to pass 
through the globes. A perforated screen at D limits 
its diameter, so that none of it shall touch the walls 
of the vessel, though the thermometer-bulb is entirely 
covered by it. A small screen at M allows the observer 
to see the shadow of the thermometer-bulb, and so to 
perceive whether the tube through which the light 
enters is properly directed. If the apparatus is mount- 
ed upon what is called an equatorial stand, like a tele- 
scope, and provided with clock-work, the whole labor 
of observation will consist merely in reading the ther- 
mometer. The difference between its temperature and 
that of the water in the surrounding shell gives the 
necessary data for calculating the intensity of the solar 
radiation at the time of reading, since the heat received 
by the thermometer from the sun and shell together 
must just equal that radiated back by the thermometer- 
bulb to the shell, after allowing for the orifices. 

Yiolle found that at noon on a fair day the ther- 
mometer of this apparatus generally stood, when ex- 
posed to the sun, from 10*5° to 12*5° centigrade (i. e., 
18-9° to 22-5° Fahr.) above the temperature of the shell 
when the latter was filled with ice-water. If it was 
filled with boiling water, as in some of his experiments, 
the difference became less by about 1° centigrade. 

The results obtained with instruments of this class, 
of course, agree very closely with those reached by the 
dynamic method. 

Instead of stating how much ice would be melted 
in a minute by a given sunbeam, we may give the num- 
ber of calories received in one minute by a square metre 
of surface exposed perpendicularly to the sun's rays at 



296 THE SUN. 

the upper surface of the atmosphere. This number, 
which may be taken as the measure of the sun's radia- 
tion, is called ''the solar constant^^ and, according to 
different experimenters, ranges from Pouillet's estimate, 
17*6, to that of Langley, which is 30*0, the latest and 
most reliable. Forbes found 28*2, and Crova and Violle, 
at a later date, 23*2 and 25*4 respectively. In the pre- 
ceding editions of tliis book it was taken at 25, but 
there is now no question that Langley's result should be 
substituted, since he has discovered an important error 
in the work of his predecessors, and by his laborious 
course of '' bolometric " observations (to be discussed a 
little later) has supplied the data for the necessary cor- 
rection. 

Instead of stating the solar constant as thirty engi- 
neeriny calories per square metre per minute, some 
prefer to state it as three small calories per square cen- 
thnetre per minute, which comes to the same thing : 
the unit of heat is a thousand times smaller, and the 
unit of surface ten thousand times less than in the first 
statement. Professor Langley himself prefers this lat- 
ter form. Those who insist on expressing all scientific 
measures in the so-called " C. G. S. system," give the 
solar constant as 0*05 small calories per square centi- 
metre per second^ which, of course, is equivalent to 
either of the other forms. 

It is not yet by any means certain that this " solar 
constant" is really constant: indeed, it is quite certain 
that it is not strictly so — tliat the amount of heat radi- 
ated by the sun must vary more or less with the changes 
which we know to occur upon its surface; at the same 
time there is no reason to suppose that the variations are 
very great. It is, however, one of the most important 
and diflScult problems of solar physics now pending to 



THE SUN'S LIGHT AND HEAT. 297 

determine the actual amount of these variations, and to 
ascertain the laws that govern them. 

By far the most difficult part of the experimental 
problem of the solar constant lies in the determination 
of the large and troublesome corrections to be applied 
on account of the absorption of the earth's atmosphei-e. 
It would take us too far to discuss the formulae and meth- 
ods of calculation which have been proposed. They are 
necessarily very complicated (those, at any rate, which 
are tolerably accurate in their results), because they have 
to take into account the meteorological conditions, espe- 
cially the hygrometric state of the air. Besides this, 
the absorption varies greatly for radiations of different 
pitch, so that the violet rays, which are photographically 
the most active, suffer more than the green and yellow, 
which are most effective in the grow^th of plants ; and 
these more than the red ; and the red, in their turn, 
much more than the low-pitched, slowly vibrating waves 
which, though invisible, are still powerful carriers of 
energy. 

Speaking loosely, it may be estimated that, at the 
sea-level, in fair weather, neither excessively moist nor 
dry, about thirty per cent, of the solar radiation is 
absorbed when the sun is at the zenith, and at least 
seventy-five per cent, at the horizon. Of the rays 
striking the upper surface of the atmosphere, between 
forty-five and fifty per cent., therefore, are generally 
intercepted in the air, even when there are no clouds. 

Of course, it does not follow that the heat absorbed 
in our atmosphere is lost to the earth. Far from it: 
the air itself becomes M^armed and communicates its 
heat to the earth ; and, since the atmosphere intercepts 
a large proportion of the heat which the earth would 
radiate into space if not thus blanketed, the temperature 



298 THE SUN. 

of tlie earth is kept much higher than it would be if 
there were no air. 

The earlier investigators sought to determine the 
amount of the necessary correction for this atmospheric 
absorption " in the gross," so to speak. That is, they 
ascertained by their experiments the whole amount of 
heat received from the sun at different apparent eleva- 
tions, when its rays had to penetrate different thick- 
nesses of air ; and from these results they attempted to 
deduce the amount of heat that would have been re- 
ceived if no air had intervened. In making the calcu- 
lation they employed a well-known formula which gives 
with practical correctness, for a homogeneous ray (all of 
one wave-length) of light or heat, the amount trans- 
mitted through any given thickness of an absorbing 
medium when we have once for all determined the per- 
centage transmitted through a stratum of the medium 
one unit thick. This percentage is called '* the coeffi- 
cient of transmission," and can be found by measuring 
the amount of this homogeneous light or heat trans- 
mitted through any two strata tliat differ considerably 
in thickness. 

Now the experimenters knew perfectly well that 
the radiant heat they were dealing with was not homo- 
geneous, but composed of rays of widely different wave- 
length : they supposed, however, that by treating the 
matter as they did they would get a sort of average 
coefficient of transmission which would be practically 
correct. 

In this they were greatly mistaken. Professor Lang- 
ley was the first to point out the error, to show that its 
correction would largely increase the estimate of the 
solar constant, and to invent the apparatus and make 
the observations necessary to bring out the truth. He 



THE SUX'S LIGHT AND HEAT. 299 

saw that it was indispensable to determine the atmos- 
pheric coefficient of transmission separately for each of 
a multitude of rays of different wave-length distributed 
all along the spectrum — the invisible portions as well as 
tlie visible — and to determine also for each ray its pro- 
portion contributed to the total amount of sunlight 
energy. For this purpose he was obliged to devise a 
heat-measurer far more delicate than any before in use, 
and with it to explore the spectrum from end to end, 
both from stations near the level of the sea and from 
the top of a lofty mountain (Mount Whitney, 15,000 
feet high). 

The new heat-measurer, which he called the " bolom- 
eter," depends upon the principle, long known and 
previously applied by Jamin and others, that the elec- 
trical resistance of a metal is increased by warming it. 
The sensitive " nerve," if we may call it so, is a little 
strip of iron or palladium about a third of an inch long, 
by j^Q wide, and i-q-^o-o thick. This is balanced against 
a similar strip placed near the first, but screened fj*om 
the heat rays to be measured. 

The two strips form the so-called '^ arms " of an elec- 
tric balance, and are connected with a delicate gal- 
vanometer, the index of which (a spot of light) moves 
whenever there is any difference of temperature be- 
tween them. The instrument wnth which Langley has 
lately been making his wonderful map of the invisible 
regions of the spectrum, indicates distinctly one mil- 
lionth of a centigrade degree. 

The strips are mounted in a little tube of hard rub- 
ber, so as to be carefully protected from all outside 
influences, except that a narrow slit in front of the 
" nerve " leaves it free to receive the rays to be ob- 
served. 



300 



THE SUN. 



The bolometer is used in connection with a large 
spectroscope, taking the place of the eyepiece : in this 
'^ spectro-bolometer," as Langley calls the combination, 
the prism, if one is used, must be of rock-salt, the only 
substance, so far as known, which freely transmits the 
invisible rays of the heat spectrum. The lenses of the 
spectroscope are also of the same material. 

For some purposes a grating can be used, but usually 
its spectrum is too feeble. 

The light is brought to the collimator slit by a mir- 
ror, and as the grating or prism is turned the spectrum 



Fig. 9S. 




LAyGLEY's Spectro- Bolometer, as used for Mapping ihe Eweegy of tub 

Prismatic Si'ECtbum. 1 

traverses the bolometer slit, and the galvanometer index 
by its motion indicates the dark lines and bright inter- 
spaces as they pass it in procession. 



m 



THE SUN'S LIGHT AND HEAT. 



301 



Fig. 98 represents one form of the instrument. The 
rays arrive through the lens L, pass through the prism 
P, and are tlien reflected back from the mirror M to 
the bolometer at B, from which the wires go to the 
galvanometer and to the battery which furnishes the 
current. 

Until quite recently the galvanometer readings had 
to be made by the eye and the records by hand, an ex- 
ceedingly tedious business, but very lately the apparatus 
has been made automatic. The galvanometer index (a 
spot of light) falls upon a sensitive plate which is car- 
ried along in precise correspondence with the motion of 
the grating or prism. The result is an irregular curve 
on the developed plate, in which dark lines of the spec- 
trum are represented by notches. 

In this way work that would have taken months by 
the old methods can be done in a single afternoon. By 
a simple process, alhO automatic, the curve can be trans- 
formed into a picture of the spectrum showing its dark 
lines and other characteristics just as does a photograph. 

In 1894 Langley published the map of the invisible 
spectrum, which we give as Fig. 99. The reader will 



Fio. 99. 




The Infra-Ked Specteum — Langlef. 

notice how very short is the visible portion of the spec- 
trum, as compared with the extensive range of rays of 
longer wave-length. The amount of energy, however, 
contained in the portion of the spectrum below (to the 
right of) the point marked 3 is extremely small, though 



302 



THE SUN. 



still sensible to a point far beyond the limit of the fig- 
ure. To a certain extent some of the salient peculiari- 
ties of the upper part of this invisible spectrum, as far 
as X on the map, had been brought to light by earlier 
investigators, especially Becquerel, Lamansky, and Ab- 
ney, the last of whom even succeeded in photographing 
a part of it. Bat Langley was the first to give us any- 
thing that could be called a map ; and in some portions 
his large-scale map, which we can not well reproduce 
here, is already fairly comparable in detail and accuracy 
with Kirchhoff's map of the visible spectrum. 

Experiments with the thermopile show that the heat 
radiated by the solar disk varies, like the light, very 
considerably from the center to the edges. The first 
observations of this kind were made by Professor Henry 
at Princeton in 1845, and have since been repeated by 
many others, Secchi and Langley especially. Accord- 
ing to Langley, the heat emitted from a point about 20'' 
from the limb is only one half that from the same extent 
of surface at the center of the disk. 







HEAT RADIATION. 




l^i^fftTipp ft*nTn PATifAi* 








Eadius = 1-00. 










Langley. 


Frost. 


Wilson. 


0-00 


100 


100-0 


100-0 


0-10 




99 


9 


99-8 


0-20 




99 


4 


99-5 


0-25 


99 






99-3 


0-30 




98 


4 


98-9 


0-40 




98 





97-2 


0-50 


95 


98 


6 


95-3 


0-60 




89 


8 


92-2 


0-70 




84 


6 


87-8 


0-75 


*86 






85-3 


0-80 




11 


9 


82-5 


0-90 




68 





72-0 


0-95 


62 






61-3 


0-98 


50 






51-5 


1-00 


... 


*(39) 


42-9 



THE SUN'S LIGHT AND HEAT. 303 

More « recently Frost at Potsdam and Wilson at 
Daramona have reinvestigated the subject a little more 
fully, and in the preceding table we give their results 
as well as Langley's. 

If we compare this table with that given on page 
281, which gives the variation of luminosity from cen- 
ter to edge of the solar disk, it is at once evident, as 
Langley was the first to point out, in 1875, that the 
absorption is, to a certain extent, selective, the short 
waves of the solar radiation being more affected than 
the long. Besides this regular variation of the radiation 
from center to edge, Secchi, in 1852, found, or thought 
he found, a notable difference between the radiation 
from the equator of the sun and that from the higher 
latitudes, the difference being at least one sixteenth 
between the equator and latitude 30°. The northern 
hemisphere he also found to be a little hotter than the 
southern. Later investigators (Langley especially) have 
failed to find any such difference ; and on the whole it 
seems probable that Secchi was mistaken, though this 
is not certain, as it w^ould be quite unsafe to assert that 
the actual condition of the sun's surface may not have 
changed between 1852 and 1876. 

In connection with the absorption of the solar at- 
mosphere, Langley has ventured some interesting specu- 
lations. After showing that variations in the number 
and magnitude of sun-spots can not directly produce 
any sensible effect upon terrestrial temperatures, he 
calls attention to the fact that even slight changes in 
the depth and density of the sun's absorbing layer would 
make a great difference ; and he raises the question 
whether we may not find here the explanation of glacial 
and carboniferous periods in the earth's history. It is 
quite certain that, were the envelope removed, the solar 



304 THE SUN. 

radiation would be about doubled, and perhaps increased 
in a much higher ratio, while any considerable increase 
of its thickness would so diminish our heat-supply as 
to give us perpetual winter. 

As yet our means of observation have not sufficed 
to detect with certainty any variations in the amount of 
heat emitted by the sun at different times. That there 
are such variations is almost certain, since the nuclei ol 
sun-spots radiate much less heat, as well as light, than 
neighboring regions of the solar surface, and the faculse 
more : this has been directly determined with the ther- 
mopile. 

Some very ingenious instruments have of late years 
been devised and constructed by the younger Angstrom 
for the purpose of recording automatically the inten- 
sity of solar radiation during an entire day, and sum- 
ming up the total amount — a process which, if carried 
out every clear day for several years at some suitable 
station, ought, after a thorough discussion, to furnish 
interesting data as to the thermal activity of the sun. 
The difficulties, however, depending on the continual 
variations in the meteorological conditions are enor- 
mous ; the truly solar variations seem to be utterly 
masked and overpowered by those that originate in our 
own atmosphere. 

As was said in the chapter upon the sun-spots, we 
are as yet entirely uncertain whether, at the time of a 
sun-spot maximum, the solar radiation is more or less 
powerful than the average. 

There has been a great deal of pretty vigorous dis- 
cussion as to the temperature of the sun, and that the 
subject is a difficult one is evident enough from the 
wide discrepancy between the estimates of the highest 



THE SUN^S LIGHT AND HEAT. 305 

autliorities. For instance, Secchi originally contended 
for a temperature of about 18,000,000° Falir. (though 
lie afterward lowered his estimate to about 250,000°) ; 
Ericsson puts the figure at 4,000,000° or 5,000,000°; 
Zollner, Spoerer, and Lane name temperatures ranging 
from 50,000° to 100,000° Fahr., while Pouillet, Yicaire, 
and Deville have put it as low as between 3,000° and 
10,000° Fahr. 

The difficulty is twofold. In the first place, the sun 
can not properly be said to have a temperature any 
more than the earth's atmosphere can. The tempera- 
ture of different portions of the solar envelope must 
vary enormously, increasing fast as w^e descend below 
or ascend above the surface. There may be a differ- 
ence of thousands of degrees between the temperatures 
of the chromosphere and of the photosphere ; and still 
other thousands between that of the photosphere and 
the depths beneath. 

We may, however, partially evade this difficulty by 
substituting as the object of inquiry the sun's effective 
temperature — i. e., instead of seeking to ascertain the 
actual temperature of different parts of the sun's sur- 
face, we may inquire what temperature would have to 
be given to a uniform surface of standard radiating 
power (a surface covered with lampblack is generally 
taken as this standard), and of the same size as the sun, 
in order that it might emit as much heat as the sun 
actually does. In this w^ay we obtain a perfectly defi- 
nite object of investigation. But the problem still re- 
mains very difficult, and has obtained as yet no entirely 
satisfactory solution. The difficulty lies in our ignor- 
ance as to the laws which connect the temperature of a 
surface with the amount of heat radiated per second. 
So long as the temperature of the radiating body does 
21 



306 THE SUN. 

not greatly exceed that of surrounding space, the heat 
emitted is very nearly proportional to the excess of 
temperature. The extremely high values of the solar 
temperature asserted by Secchi and Ericsson depend 
upon the assumption of this law (knovt^n as Newton's) 
of proportionality between the heat radiated and the I: 
temperature of the radiating mass — a law which direct 
experiment proves to be untrue as soon as the tempera- 
ture rises a little. In reality, the amount of heat radi- 
ated increases much faster than the temperature. 

More than fifty years ago the French physicists, 
Dulong and Petit, by a series of elaborate experiments, 
deduced an empirical formula, which answered pretty 
satisfactorily for temperatures up to a dulhred heat. 
By applying this formula, Pouillet, Vicaire, and others 
arrived at the low solar temperatures assigned by them. 
It is, however, evidently unsafe to apply a purely em- 
pirical formula to circumstances so far outside the range 
of the observations upon which it was founded, and, in 
fact, within a few years several experimenters, Rosetti 
especially, have shown that it needs modification, even 
in the investigation of artificial temperatures like that 
of the electric arc. Eosetti, from his observations, has 
deduced a different law of radiation, and by its appli- 
cation finds 10,000° Cent., or 18,000° Fahr., as the effec- 
tive temperature of the sun — a result which, all things I 
considered, seems to the writer more reasonable and bet- 
ter founded than any of the earlier estimates. Rosetti 
considers that this is also pretty nearly the actual tem- 
perature of the upper layers of the photosphere. The | 
radiating power of the photospheric clouds, to be sure, 
can hardly be as great as that of lampblack ; but, on the 
other hand, their radiation is supplemented by that of 
other layers, both above and below. 



THE SUN'S LIGHT AND HEAT. 307 

Still more recently (in 1892) Le Chatelier has de- 
duced an effective temperature of 7,600° Cent, (or about 
13,700 Fahr.) by a study of the intensity of certain red 
rays of the sun compared with the intensity of the same 
rays in the radiations of certain bodies heated to lumi- 
nosity. 

Still later Wilson and Gray, by a very elaborate in- 
vestigation, which, on the whole, seems entitled to take 
the precedence over all competitors so far, have obtained 
as a result 8,000° ^ Cent., or 11,400° Fahr. Their appa- 
ratus and methods were dift'erent from any previously 
employed : as a measurer of the radiation they used the 
" radio-micrometer " of Boys, which combines thermo- 
pile and galvanometer in one, and possibly surpasses in 
sensitiveness even the "bolometer" of Langley. For 
the determination of temperatures, by which to inter- 
pret the radiation measures, they used a modified form 
of Joly's meldometer, in which a strip of platinum is 
raised to any desired temperature short of melting by 
a current from a storage battery or dynamo. The ap- 
paratus utilizes the inventions and resources of the new- 
est scientific art, and would have been impossible a 
dozen years ago. 

It is wortli noting that their observations disprove 
Dulong and Petit's law of radiation, and seem to con- 
firm the much simpler one proposed by Stephan of 
Vienna, and theoretically deduced by Boltzmann as a 
consequence of the electro-magnetic theory of light. 
The formula is simply this, E = <2 X T* : in which K is 
the intensity of the radiation in imits of energy, <^ is a 
constant coefficient, depending on the nature of the body 

* The highest artificial temperatures are supposed to fall short of 
2,500° Cent. Possibly this figure may now be exceeded in some of the 
electric furnaces. 



308 THE SUN. 

and units of energy and temperature employed, while 
T is the temperature of the body above the absolute 
zero, and is equal to the centigrade temperature with 
273° added. 

If the effective temperature of the sun were doubled 
w^e should receive sixteen times as much heat, and ten 
thousand times as much if its temperature were in- 
creased only ten times. 

Scheiner, of Potsdam, has shown spectroscopically 
that the temperature of the sun's reversing layer is al- 
most certainly intermediate between that of the electric 
arc and the much higher temperature of the Leyden-jar 
spark. In the spectrum of magnesium there are two 
lines at \ 4,482 and X 4,352 which are strikingly con- 
trasted in behavior. The former is strong in the spark- 
spectrum but hardly visible in that of the arc, while with 
4,352 the reverse is true ; and Scheiner shows that the 
difference depends upon temperature. Now in the solar- 
spectrum 4,352 is a conspicuous dark line, and the other 
is missing ; whence the inference that the magnesium 
vapor in the reversing layer is about as hot as the electric 
arc, and the photosphere below, of course, still hotter. 

Besides the data as to the intensity of the solar 
temperature obtained by the methods above mentioned, 
we have also direct evidence of a very impressive sort. 
When heat is concentrated by a burning-glass, the tem- 
perature at the focus can not rise above that of the 
source of heat, the effect of the lens being simply to 
move the object at the focus virtually toward the sun ; 
so that, if we neglect the loss of heat by transmission 
through the glass, the temperature at the focus should 
be the same as that of a point placed at such a distancoi 
from the sun that the solar disk would seem just a| 
large as the lens itself viewed from its own focus. 



TnE SUN'S LIGHT AND HEAT. 309 

The most powerful lens yet constructed thus virtu- 
ally transports an object at its focus to within about 
two hundred and tifty thousand miles of the sun's sur- 
face, and in this focus the most refractory substances- 
platinum, lire-clay, the diamond itself — are either in- 
stantly melted or dissipated in vapor. There can be no 
doubt that, if the sun were to come as near us as the 
moon, the solid earth would melt like wax. 

We have spoken, a few pages back, of Professor 
Langley's experimental comparison between the brill- 
iance of the solar surface and that of the metal in a 
Bessemer converter. At the same time he made meas- 
urements of the heat by means of a thermopile, and 
found the heat radiation of the solar surface to be more 
than eighty-seven times as intense as that from the sur- 
face of the molten metal. It will be recalled that the 
experiment only sets a lower limit to the solar radiation, 
so that it is altogether probable that, were all the neces- 
sary corrections determined and applied, the ratio would 
be increased from eighty-seven to at least a hundred, 
and perhaps to a hundred and fifty. Ericsson, in 1872, 
made a somewhat similar comparison in a different and 
exceedingly ingenious manner. He floated a calorime- 
ter containing about ten pounds of water upon the sur- 
face of a large mass of molten iron, by means of a raft 
of fire-brick. The calorimeter was raised a little above 
the surface, and the water contained was kept in circu- 
lation by suitable mechanism. He found that the radia- 
tion of the metal was a trifle over two hundred and fifty 
calories per minute for each square foot of surface. 
This is equivalent to twenty-seven hundred and ninety 
calories to the square metre, and is only ^^ of the sun's 
emission. He estimated the temperature of the metal 
at 3;,000'' Fahr., or IjOSS"^ Cent. Professor Langley, in 



310 THE SUN. 

his experimentj estimated the temperature of the Bes- 
semer metal much higher — superior, in fact, to the tem- 
perature of melting platinum, which is usually consid- 
ered to be about 2,000° Cent. He bases this conclusion 
upon the fact that platinum wire, stretched above the 
mouth of the converter, or dipped into the issuing 
stream, was immediately melted. Since, however, iron 
and its vapor attack platinum much in the same way as 
mercury and its vapor attack gold, there may be some 
doubt as to the correctness of his estimate. The same 
conclusions as to the intensity of the solar temperature 
follow from investigations by Soret and others as to 
the penetrating power of the sun's rays, and from a 
comparison with artificial sources of heat in rospoct to 
the relative proportion of the rays of different w\ave- 
lengths in the total radiation. A body of low tempera- 
ture emits an enormous proportion of slow-swinging, 
invisible vibrations, while, as the temperature rises, the 
shorter waves become proportionally more and more 
abundant. Thus, in the composition of a body's radia- 
tion, we get some clew to its temperature. Hitherto 
all such tests concur in putting the sun's temperature 
high above that of any known terrestrial flame. 

And now we come to questions like these : How is 
such a heat maintained? How long has it lasted al- 
ready? How long will it continue? Are there any 
signs of either increase or diminution? — questions to 
which, in the present state of science, only somewhat 
vague and unsatisfactory replies are possible. 

As to progressive changes in the amount of the solar 
heat it can be said, however, that there is no evidence 
of anything of the sort since the beginning of authentic 
records. There have been no such changes in the dis- 
tribution of plants and animals within the last two thou- 



THE SUN'S LIGHT AND HEAT. 311 

sand years, as must have occurred if there had been, 
within this period, any appreciable alteration in the 
heat received from the sun. So far as can be made 
out, with few and slight exceptions, the vine and olive 
grow just where they did in classic days, and the same 
is true of the cereals and the forest-trees. In the re- 
moter past there have been undoubtedly great change3 
in the earth's temperature, evidenced by geological 
records — carboniferous epochs, when the temperature 
was tropical in almost arctic latitudes, and glacial pe- 
riods, when our now temperate zones were incased in 
sheets of solid ice, as northern Greenland is at present. 
Even as to these changes, however, it is not yet certain 
whether they are to be traced to variations in the amount 
of heat emitted by the sun, or to changes in the earth 
herself, or in her orbit. So far as observation goes, we 
can only say that the outpouring of the solar heat, amaz- 
ing as it is, appears to have gone on unchanged through 
all the centuries of human history. 

What, then, maintains tlie fire ? It is quite certain, 
in the first place, that it is not a case of mere combus- 
tion. As has been said, only a few pages back, it has 
been shown that, even if the sun were made of solid 
coal, burning in pure oxygen, it could only last about 
six thousand years : it would have been nearly one third 
consumed since the beginning of the Christian era. 
Nor can the source of its heat lie simply in the cooling 
of its incandescent mass. Huge as it is, its temperature 
must have fallen more than perceptibly within a thou- 
sand years if this were the case. 

Many difi^erent theories have been proposed, two of 
which now chiefly occupy the field. One of them finds 
the chief source of the solar heat in the impact of 
meteoric matter, the other in the slow contraction of the 



312 THE SUK 

sun. As to the first, it is quite certain that a part of 
the solar heat is produced in that way ; but the question 
is whether the supply of meteoric matter is sufficient 
to account for any great proportion of the whole. As 
to the second, on the other hand, there is no question 
as to the adequacy of the hypothesis to account for the 
whole supply of solar heat ; but there is as yet no direct 
evidence whatever that the sun is really shrinking. 

The basis of the meteoric theory is simply this : If 
a moving body be stopped, either suddenly or gradually, 
a quantity of heat is generated which may be expressed, 

in calories, by the formula , in which r)i is the mass 

o,oo9 

of the body, in kilogrammes, and v its velocity, in me- 
tres per second. A body weighing 8,339 kilogrammes, 
and moving one metre per second, would, if stopped, 
develop just one calory of heat — i. e., enough to heat 
one kilogramme of water from freezing-point to 1° 
Cent. If it were moving five hundred metres per 
second (about the speed of a cannon-ball), it would pro- 
duce two hundred and fifty thousand times as much 
heat, or enough to raise the temperature of a mass of 
water equal to itself about 30° Cent. If it were mov- 
ing, not five hundred metres per second, but about seven 
hundred thousand (approximately the velocity with 
which a body would fall into the sun from any planet- 
ary distance), the heat produced would be 1,400 x 1,400, 
or nearly two million, times as great — sufficient to bring 
a mass pf matter many thousand times greater than itself 
to most vivid incandescence, and immensely more than 
could be produced by its complete combustion under 
any conceivable circumstances. Lord Kelvin (formerly 
Sir William Thomson) has calculated the amount of 
heat which would be produced by each of the planets 



THE SUN'S LIGHT AND HEAT. 313 

in falling into the sun from its present orbit. The re- 
sults are as follows, the heat produced being expressed 
by the number of years and days through which it 
would maintain the sun's present expenditure of en- 
ergy: 

Years. Days. 

Mercury 6 219 

Venus 83 326 

Earth 95 19 

Mars 12 259 

Jupiter 32,254 

Saturn 9,652 

Uranus 1,610 

Neptune 1,890 

Total 45,604 

That is, the collapse of all the planets upon the sun 
would generate sufficient heat to maintain its supply for 
nearly forty-six thousand years. A quantity of matter 
equal to only about one one-hundredth of the mass of 
the earth, falling annually upon the solar surface, would, 
therefore, maintain its radiation indefinitely. Of course, 
this increase of the sun would cause an acceleration of 
the motion of all the planets — a shortening of their 
periods. Since, however, the mass of the sun is three 
hundred and thirty thousand times that of the earth, 
the yearly addition would be only one thirty-three-mill- 
ionth of the whole, and it w^ould require centuries to 
make the effect sensible. The only question, then, is, 
whether any such quantity of matter can be supposed 
to reach the sun. While it is impossible to deny this 
dogmatically, it, on the whole, seems improbable, for 
astronomical reasons. In the first place, if meteoric 
matter is so abundant, the earth ought to encounter 
much more of it than she does ; enough, in fact, to 
raise her temperature above that of boiling water. 



314 THE SUN. 

Then, again, if so large a quantity of matter annually 
falls upon the solar surface, it is necessary to suppose 
a vastly greater quantity circulating around the sun 
between it and the planet Mercury. The process by 
which the orbit of a meteoric body is so changed as 
to make it enter the solar atmosphere is a very slow 
one, so that only a very small proportion of the w^hole 
could be caught in any given year. Now, if there were 
near the sun any considerable quantity of meteoric 
matter — anything like the mass of the earth, for in- 
stance — it ought to produce a very observable effect 
upon the motions of the planet Mercury, an effect not 
yet detected,"^ For this reason astronomers generally, 
while conceding that a portion, and possibly a consider- 
able fraction, of the solar heat may be accounted for by 
this hypothesis, are disposed to look further for their 
explanation of the principal revenue of solar energy. 
They find it in the probable slow contraction of the 
sun's diameter, and the gradual liquefaction and solidi- 
fication of the gaseous mass. The same total amount 
of heat is produced wdien a body moves against a resist- 
ance which brings it to rest gradually as if it had fallen 
through the same distance freely and been suddenly 
stopped. If, then, the sun does contract, heat is neces- 
sarily produced by the process, and that in enormous 
quantity, since the attracting force at the solar surface 
is more than twenty-seven times as great as gravity at 
the surface of the earth, and the contracting mass is so 
immense. 

* Levcrricr considered that he had detected in the motions of Mercury 
an irregularity of the kind indicated, but much smaller. It was such, 
according to his calculations, as would be accounted for by the action of 
one or several planets whose aggregate mass should be much less than 
that of the earth. This was the basis on which he founded his strong 
belief in the existence of the intra-Mercurial planet Vulcan. 



THE SUN'S LIGHT AND HEAT. 315 

In this process of contraction, each particle at the 
surface moves inward by an amount equal to the whole 
diminution of the solar radius, while a particle below 
the surface moves less, and under a diminished gravi- 
tating force ; but every particle in the whole mass of 
the sun, excepting only that at the exact center of the 
globe, contributes something to the evolution of heat. 
To calculate the precise amount of heat developed, it 
would be necessary to know the law of increase of the 
sun's density from the surface to the center ; but Helm- 
holtz, who first suggested the hypothesis, in 1853, has 
shown that, under the most unfavorable suppositions, a 
contraction ^ in the sun's diameter of about three hun- 
dred feet a year — a mile in a little more than seventeen 
years — would account for its whole annual heat-emis- 
sion. This contraction is so slow that it would be quite 
imperceptible to observation. It would require seven 
thousand years to reduce the diameter a single second of 
arc, and nothing less would be certainly detectable. 

Of course, if the contraction is more rapid than this, 
the mean temperature of the sun must be actually rising, 
notwithstanding the amount of heat it is losing. Obser- 
vation alone can determine whether this is so or not. 

If the sun were wholly gaseous, we could assert 
positively that it must be growing hotter; for it is a 
most curious (and at first sight paradoxical) fact, first 
pointed out by Lane in 1870, that the temperature of 
a gaseous body continually I'ises as it contracts from 
loss of heat. By losing heat it contracts, but the heat 
generated by the contraction is more than suflicient to 
keep the temperature from falling. A gaseous mass 
losing heat by radiation must, therefore, at the same 

* These figures have been altered to correspond with Langley's value 
of the solar constant. 



316 THE SUN. 

time grow both smaller and hotter, until the density 
becomes so great that the ordinary laws of gaseous ex- 
pansion reach their limit and condensation into the 
liquid form begins. The siin seems to have arrived 
at this point, if indeed it were ever wholly gaseous, 
which is questionable. At any rate, so far as we can 
now make out, the exterior portion— i. e., the photo- 
sphere — appears to be a shell of cloudy matter, precip- 
itated from the vapors which make up the principal 
mass, and the progressive contraction, if it is indeed a 
fact, must result in a continual thickening of this shell 
and the increase of the cloud-like portion of the solar 
mass. 

This change from the gaseous to the liquid form 
must also be accompanied by the liberation of an enor- 
mous quantity of heat, sufficient to materially diminish 
the amount of contraction needed to maintain the solar 
radiation. 

Evidently if this theory of the source of the solar 
heat is correct, it follows that in time it must come to 
an end ; and looking backward we see that there must 
also have been a beginning. Time was when there was 
no such solar heat as now, and the time must come when 
it will cease. 

We do not know enough about the amount of solid 
and liquid matter at present in the sun, or of the nature 
of this matter, to calculate the future duration of the 
sun with great exactness, though an approximate esti- 
mate can be made. The problem is a little complicated, 
even on the simplest hypothesis of purely gaseous con- 
traction, because as the sun shrinks the force of gravity 
increases, and the amount of contraction necessary to 
generate a given amount of heat becomes less and less ; 
but this difficulty is easily met by a skillful mathema- 



THE SUVS LIGHT AND HEAT. 317 

tician. According to Newcomb, if the sun maintains 
its present radiation it will have shrunk to half its pres- 
ent diameter in abont five million years at the long- 
est. As it must, when reduced to this size, be eight 
times as dense as now, it can hardly then continue to 
be mainly gaseous, and its temperature must have begun 
to fall. Newcomb's conclusion, therefore, is that it is 
hardly likely that the sun can continue to give sufficient 
heat to support life on the earth (such life as we now 
are acquainted with, at least) for ten million years from 
the present time. 

It is possible to compute the past of the solar his- 
tory upon this hypothesis somewhat more definitely 
than the future. The present rate of contraction being 
known, and the law of variation, it becomes a purely 
mathematical problem to compute the dimensions of 
the sun at any date in the past, supposing its heat-radi- 
ation to have remained unchanged. Indeed, it is not 
even necessary to know anything more than the present 
amount of radiation, and the mass of the sun, to com- 
pute how long the solar fire can have been maintained, 
at its present intensity, by the process of condensation. 
No conclusion of geometry is more certain than that 
the contraction of the sun from a diameter even many 
times larger than that of Neptune's orbit to its present 
dimensions, if such a contraction has actually taken 
place, has furnished about eighteen million times as 
much heat as the sun now supplies in a year ; and there- 
fore that the sun can not have been emitting heat at 
the present rate for more than that length of time, if its 
heat has really been generated in this manner. If it 
could be shown that the sun has been shining as now for 
a longer time than that, the theory would be refuted ; 
but if the hypothesis be true, as it probably is in the 



318 THE SUN. 



1 



main, we are inexorably shut up to the conchision tliat 
the total life of the solar system, from its birtli to its 
death, is included in some such space of time as tliirty 
million years. No reasonable allowances for tlie fall of 
meteoric matter, based on what we are now able to ob- 
serve, or for the development of heat by liquefaction, 
solidification, and chemical combination of dissociated 
vapors, could raise it to sixty million. 

At the same time, it is obviously impossible to assert 
that there has been no catastrophe in the past — no col- 
lision with some wandering star, endued, as Croll has 
supposed, like some of those we know of now in the 
heavens, with a velocity far surpassing that to be ac- 
quired by a fall even from infinity, producing a shock 
which might in a few hours, or moments even, restore 
the wasted energy of ages. Neither is it wholly safe to 
assume that there may not be ways, of which we yet 
have no conception, by which the energy apparently 
lost in space may be returned, at least in part, and so 
the evil day of the sun's extinction may be long post- 
poned. 

In 1882 Dr. C. W. Siemens, of London, proposed a 
new theory of the solar energy much in this line, and 
the scientific eminence of its author secured it most re- 
spectful consideration and discussion. Although it was 
soon abandoned as untenable, on account of want of 
evidence as to some of its fundamental assumptions, 
and fatal objections to it from astronomical consider- 
ations, it is so suggestive and instructive that we have 
concluded to retain the statement and discussion of it 
that was given in a supplementary note to the second 
edition of this book a dozen years ago. 

" The fundamental conditions " of Dr. Siemens's 
theory are the following, in his own words : 



THE SUX'S LIGHT AND HEAT. 319 

"1. That aqueous vapor and carbon compounds are present 
in stellar and interplanetary space. 

'^2. That these gaseous compounds are capable of being disso- 
ciated (decomposed into their elements; by radiant solar energy 
while in a state of extreme attenuation. 

''3. That these dissociated vapors are capable of being com- 
pressed into the solar atmosphere by a process of interchange 
with an equal amount of reassociated vapors, the interchange 
being effected by the centrifugal action of the sun itself." 

Granting these conditions, he argues that the solar 
heat is produced by the recombination of the elemental 
gases contained in a current which is continually drawn 
in upon the region of the sun's poles ; this current be- 
ing produced by the rotation of the sun, which acts like 
a gigantic fan-wheel, throwing off the adherent gases 
at its equator, and thus continually removing the prod- 
ucts of the combustion^ and redistributing them through 
space. 

Moreover — and this is the point of the theory upon 
which he puts special emphasis — he teaches that these 
compound gases resulting from the combustion inter- 
cept the solar heat not received by the planets (heat 
which, from the human point of view, would otherwise 
be wasted), and utilize it in their own decomposition ; 
thus the solar fire is made to prepare its own fuel from 
the ashes of its own furnace, and an explanation is found 
for its enduring constancy. 

While, for reasons soon to be stated, we can not ac- 
cept this theory, it may be said, in the first place, that 
there is nothing absurd in it. It is not to be put into 
the category of the speculations which explain gravita- 
tion and the planetary motions by electric vortices, or 
any similar nonsense. 

If space is filled with composite vapors, and if rays 
of light and heat can decompose them again into their 



320 THE SUN. 

elements, then, to some extent, the theory not only may 
be but must be true. A hot revolving globe, moving 
in a space filled with such vapors, must necessarily pro- 
duce such currents as Dr. Siemens indicates, and must 
maintain a continual fire upon its surface : the question 
would only be how great and how hot a fire. 

I^ow, as to Dr. Siemens's fundamental hypotheses. 
Probably no one will care to deny the possible existence 
of some gaseous matter in space, though it seems much 
more probable that w^hat matter is scattered about be- 
tween the planets is mostly in the form of little solid 
grains, such as we see from time to time in shooting-stars. 

But an interplanetary atmosphere of any sensible 
density is, we think, irreconcilable with the observed 
motions of planets, and especially of comets. Dr. Sie- 
mens suggests -3 oVo" ^f ^^ density of the earth's atmos- 
phere as a possible maximum, without indicating what 
the minimum might be. But, in order to supply the 
requisite amount of material to keep up the sun's heat, 
the density of the interplanetary atmosphere would 
have to be somewhere from y^ to To'ro'o 0" ^^ ^^^ d^^" 
sity of the air at the earth's surface."^ 

* In an article, contained in the " Nineteenth Century " for April, 
1882, Dr. Siemens, meeting some objections that had been made to his 
theory, shows that a current of mingled gases, containing five per cent, 
of uncombined hydrogen and marsh-gas, and ninety-five per cent, of oxy- 
gen, nitrogen, and neutral gas, would by its combustion account for the 
whole supply of solar heat, if at the sun's surface the density were the 
same as that of the earth's atmosphere, and the velocity a hundred feet 
per second. Taking this estimate, and assuming that the inrushing cur- 
rent continues to be perceptible at a distance even so great as fifty mill- 
ions of miles, and that its filaments move toward the sun in converging 
lines, we find that the interplanetary atmosphere must there have a den- 
sity of about xroiiTT of the air at the earth's surface. If we suppose the 
indraught to vanish at a less distance from the sun, the density must be 
greater. 



I 




THE SUN'S LIGHT AND HEAT. 321 

Now the resistance of an atmosphere, of even the 
density mentioned, would be very serious upon bodies 
moving from fifty to one hundred times faster than 
cannon-balls, and the effect could not fail to be felt in 
a considerable retardation and a consequent shorten- 
ing of their periods. So far as can be judged from 
the observed behavior of Encke's comet, the density 
of the interplanetary medium can not well exceed 
^o,7roo,oio,o7ro,ooo- ^^ ^^^^ earth's atmosphere. (See 
Harkness's discussion of the subject in the Washington 
astronomical observations for 1870.) 

Siemens indeed remarks that, '^ assuming that the 
matter filling space is an almost perfect fluid not lim- 
ited by border surfaces, it can be shown that the retar- 
dation by friction through such an attenuated medium 
would be very slight indeed, even at planetary veloci- 
ties." But, so far as experiments go, the rarefaction of 
a gas does not tend to bring it more nearly to the con- 
dition of a perfect fluid ; on the other hand, it seems 
to approach rather to the condition of a congeries of 
minute isolated pellets flying through space : witness 
the phenomena of the radiometer and Crookes's tubes. 

Another, and it seems to us quite as serious an ob- 
jection to the theory, lies in the fact that it limits the 
temperature of the solar surface to that corresponding 
t) the dissociation point of the gaseous compounds 
there formed ; and this dissociation point for the hydro- 
carbons and the vapor of water, even granting a con- 
siderable pressure at the solar surface, is not above 
6,000° to 8,000° Fahr., which is much below the tem- 
perature of the sun's surface indicated by all the most 
recent determinations. 

Furthermore, if the absorption of radiant energy 
within the limits of the solar system really amounts to 
22 



322 THE SUN. 



1 



anything sensible, the stars ought to be quite invisible, 
or at any rate no heat should reach us from them. 

As to the chemical assumption that the hydrocarbon 
compounds when greatly rarefied can be dissociated by 
the action of the sun's rays, we believe that no evi- 
dence has been found of such an effect. 

And yet one almost regrets that the theory can not 
be accepted, for it would remove some very serious dif- 
ficulties that now embarrass the problem of the evolu- 
tion of our planetary system. The accepted contraction 
theory of Helmholtz certainly appears to allow too little 
time for the sun's lifetime of radiant activity to be con- 
sistent with a reasonable explanation of the process by 
which the present state of things has come about. 

While we have mentioned only three theories of the 
solar heat, the reader will understand that a multitude 
have been proposed and rejected, some as absurd and 
others as inadequate. To the former class belong the 
speculations of those who liken the sun to the armature 
of a dynamo or the whirling plate of an " influence ma- 
chine," forgetting that in both these cases the energy 
radiated as light and heat must be derived ultimately 
from the sun's energy of rotation ; and a simple calcu- 
lation shows that this energy of rotation is not sufii- 
cient to maintain the radiation for even one hundred 
and fifty years. 

Those theories, on the other hand, that seek to ac- 
count for the solar heat as the simple cooling of an 
incandescent body, like a red-hot ball of metal, or by 
tlie " combustion " of solar material, in the chemical 
sense of the word, or by the simple condensation of 
vapors into clouds and the liberation of the so-called 
latent heat of vaporization — these all, like the meteoric 
theory, are utterly inadequate. 



CHAPTEK IX. 

SUMMARY OF FACTS, A^^D DISCUSSIOI^ OF TEE CONSTITUTION 

OF THE SUN. 

Table of Numerical Data. — Constitution of Sun's Nucleus. — ^Peculiar 
Properties of Gases under High Temperature and Pressure. — Char- 
acteristic Differences between a Liquid and a Gas. — Constitution of 
the Photosphere and Higher Regions of the Sun's Atmosphere. — 
Professor Hastings's Theory. — Pending Problems of Solar Physics. 

It may be well to collect into a brief summary the 
principal facts and conclusions of the preceding pages, 
presenting them in a single comprehensive view. We 
give first, therefore, a table of the statistics of the sun 
— the facts which can be stated in numbers : 

Solar parallax (equatorial horizontal), 8*80'^ ± 0*02''. 

Mean distance of the sun from the earth, 92,885,000 miles; 
149,480,000 kilometres. 

Variation of the distance of the sun from the earth between Janu- 
ary and June, 3,100,000 miles; 4,950,000 kilometres. 

Linear value of 1" on the sun's surface, 450*3 miles ; 724*7 kilo- 
metres. 

Mean angular semidiameter of the sun, 16' 02'0'' ± TO". 

Sun's linear diameter, 866,400 miles ; 1,394,300 kilometres. (This 
may, perhaps, be variable to the extent of several hundred 
miles.) 

Ratio of the sun's diameter to the earth's, 109 5. 

Surface of the sun compared with the earth, 11,940. 

Yolnme, or cubic contents, of the sun compared with the earth, 
1,305,000. 

Mass, or quantity of matter, of the sun compared with the earth, 
331,000 ± 1,000. 

Mean density of the sun compared with the earth, 0*253. 

Mean density of the sun compared with water, 1*406. 



(CarringtoD.) 



324 THE SUK 

Force of gravity on the sun's surface compared with that on the 
earth, 27-6. 

Distance a body would fall in one second, 444*4 feet; 135*5 metres. 

Inclination of the sun's axis to the ecliptic, 7° 15\ 

Longitude of its ascending node, 74°. 

Date when the sun is at the node, June 4-5. 

Mean time of the sun's rotation, 25-38 days. 

Time of rotation of the sun's equator, 25 days. 

Time of rotation at latitude 20°, 25*75 days. 

Time of rotation at latitude 30°, 26*5 days. 

Time of rotation at latitude 45°, 27 "5 days. 

(These last four numbers are somewhat doubtful, the formulaB 

of various authorities giving results differing by several hours in 

some cases.) 

Linear velocity of the sun's rotation at his equator, 1*261 miles 
per second ; 2*028 kilometres per second. 

Total quantity of sunlight, 1,575,000,000,000,000,000,000,000,000 
candles. 

Intensity of the sunlight at the surface of the sun, 190,000 times 
that of a candle-flame ; 5,300 times that of metal in a Bessemer 
converter; 146 times that of a calcium-light ; 3*4 times that 
of an electric arc. 

Brightness of a point on the sun's limb compared with that of a 
point near the center of the disk, 25 per cent. 

Heat received per minute from the sun upon a square metre, per- 
pendicularly exposed to the solar radiation, at the upper sur- 
face of the earth's atmosphere {the solar constant), 30 calories. 

Heat-radiation at the surface of the sun, per square metre per 
minute, 1,340,000 calories. 

Thickness of a shell of ice which would be melted from the sur- 
face of the sun per minute, 58*2 feet; or 17*7 metres. 

Mechanical equivalent of the solar radiation at the sun's surface, 
continuously acting, 131,000 horse power per square metre; 
or 12,000 (nearly) per square foot. 

EfiPective temperature of the solar surface, about 10,000° Cent., 
or 18,000° Fahr. (according to Rosetti); about 8,000° Cent., 
or 14,400° Fahr. (according to Wilson and Gray). 

Of course, it hardly need be repeated here that the 
figures relating to the light and heat of the sun are 



SUMMARY OF FACTS, ETC. 325 

much less reliable tlmn those which refer to its distance, 
dimensions, mass, and attracting power. 

Fig. 100 is intended to present to the eye, more 
clearly than any mere description, the constitution of 
the sun, and the relation of the different concentric 
shells or envelopes as conceived by the writer. 

The picture is an ideal section through the centero 
The black disk represents the inner nucleus, which is 
not accessible to observation, its nature and constitution 
being a mere matter of inference. The white ring sur- 
rounding it is the photosphere, or shell of incandescent 
cloud which forms the visible surface. The depth, or 
thickness, of this shell is quite unknown ; it may be 
many times thicker than represented, or possibly some- 
what thinner. Nor is it certain whether it is separated 
from the inner core by a definite surface, or whether, 
on the other hand, there is no distinct boundary between 
them. 

The outer surface of the photosphere, however, is 
certainly pretty sharply defined, though very irregular, 
rising at points into faculae, and depressed at others in 
spots, as shown in the figure. 

Immediately above this lies the so-called " reversing 
stratum," in which the Fraunhofer lines originate. It 
is to be noted, however, that the gases which compose 
this stratum do not merely overlie the photosphere, but 
they also fill the interspaces between the photospheric 
clouds, forming the atmosphere in which they float, 
and an attempt has been made to indicate this fact in 
the diagram. (See page 326.) 

Above the '' reversing stratum" lies the scarlet 
chromosphere, with prominences of various forms and 
dimensions rising high above the solar surface ; and 
over, and embracing all, is the coronal atmosphere and 



326 



THE SUN. 



the mysterious radiance of clouds, rifts, and streamers, 
fading gradually into the outer darkness. 

At the center of the sun the earth is represented in 
its true relative dimensions — -rh^ of the three inches 



Fig. 100. 




which is taken as the scale of the sun's diameter. This 
scale reduces our globe to a little dot only -J^ of an inch 
across. Around it, at its proper distance, is drawn the 
orbit of the moon, still far within the photosphere, the 
moon herself being fairly represented by any one of 



SUMMARY OF FACTS, ETC. 327 

the minute points which make up the dotted line tliat 
indicates her path. 

The central nucleus is made black in the picture, 
simply for convenience, and not with any purpose to 
indicate that the matter which composes it is cooler or 
even less brilliantly luminous than the photosphere. It 
is quite probable, indeed, that this central core (which 
contains certainly more than nine tenths of the whole 
mass of the sun) is purely gaseous, and it is of course 
true that, at a given temperature and jpressure^ a gaseous 
mass has a lower radiating power, and is less luminous, 
than a mass of clouds, such as those which constitute 
the photosphere. But, on the other hand, both com- 
pression and increase of temperature rapidly raise the 
radiating power of a gas ; and it is highly probable that, 
at no very considerable depth, the growing pressure and 
heat may more than equalize matters, and render the 
central nucleus as intensely bright as the surface of the 
sun itself. 

At the upper surface of the photosphere, however, 
and all through it, indeed, the uncondensed gases are 
dark as compared with the droplets and crystals which 
make up the photospheric clouds. Here the pressure 
and temperature are lowered, so that the vapors give 
out no longer a continuous but a bright-line spectrum, 
whenever we get a chance to see them, against a non- 
luminous background ; and, when the intenser light from 
the liquid and solid particles of the photosphere shines 
through these vapors, they rob it of the corresponding 
rays, and produce for us the familiar dark-lined spec- 
trum of ordinary sunlight. 

It is, perhaps, hardly necessary to state again the 
reasons for believing the great body of the sun to be 
gaseous ; the argument depends upon the enormous 



328 THE SUN. 

heat at the surface, which keeps the solar atmosphere 
charged with the vapors of our familiar metals, and the 
fact that the mean density of the sun is so low (only- 
one and one fourth times that of water), that it is quite 
impossible that any of the substances which we have 
reason to believe to exist in the sun could have the 
solid, or even the liquid, form through any considerable 
portion of its mass. That is to say, if any large propor- 
tion of the whole were composed of solid or liquid iron, 
titanium, magnesium, etc., the density would be far 
greater than it really is ; and, since the temperature, at 
the surface even, where there is free radiation and ex- 
posure to the cold of space, is so high as to keep these 
bodies in the state of vapor, it is not likely that, at 
greater depths, it is low enough to permit their lique- 
faction or solidification. 

And yet the theory that they are in a gaseous state 
is not free from difficulties. A few years ago it would 
have been urged with great plausibility that, under the 
enormous pressure due to the weight of the superin- 
cumbent mass acted upon by the solar gravity — nearly 
twenty-eight times that of the earth, it is to be remem- 
bered — any gas whatever must be liquefied at no very 
great depth below the surface. 

Even on the earth, for example, the density of the 
air decreases one half for every three and a half miles 
of elevation, and it ough-t to increase in a similar pro- 
portion for every three and a half miles of descent be- 
low the sea-level, if we drop for a moment considerations 
relative to temperature. Since water is about seven 
hundred and seventy times as heavy as the air at the 
earth's surface, it follows, therefore, that at the bottom 
of a shaft thirty-five miles deep the air would be more 
dense than water^ if of the same temperature as at 



SUMMARY OF FACTS, ETC. 329 

the surface ; and, before a depth of fifty miles were 
reached, it would become denser than gold, unless it 
had first liquefied, and so become less compressible. 
If we take account of the slight decrease of the force of 
gravity as we go below the earth's surface, and assume 
that the temperature increases, even at the rate of 100° 
Fahr. for each mile of descent, the results will be modi- 
fied, but not materially changed in character. It would 
merely be necessary to go some ten miles deeper to 
reach the same result. 

Now, at the sun, where the action of gravity is so 
much more intense, it is evident that, anless the tern- 
perature rises very rapidly ielow the surface^ or unless 
liquefaction supervenes, the density of gases must in- 
crease so fast that the mean density of the mass — if the 
sun be really gaseous — must be vastly greater than that 
of any known metal. 

But liquefaction, as we now know, can not take 
place under the circumstances. The researches of An- 
drews and his successors have shown that to liquefy 
a gas two things must go together — increase of pressure 
and diminution of temperature. For each gas there is 
a so-called '' critical temperature," and, so long as the 
temperature does not fall below this point, no pressure 
whatever can reduce the gas to the liquid form. When 
the temperature has fallen below it, then pressure alone 
will produce the desired effect, and, if the temperature is 
very low, only a slight degree of pressure will be needed. 
Now on, or in, the sun the temperature can not be 
supposed to be below the " critical points " of several 
of the gases found there, and hence, as has been said, 
their liquefaction is out of the question. Those, there- 
fore, who are unwilling to admit a sufficient increase of 
temperature with increasing depth below the solar sur- 



330 THE SUN. 

face, have been disposed to hold that the central por^ 
tions of the sun are not composed, to any great extent, 
of the same elements which the spectroscope reveals to 
us in the solar atmosphere, but of some different un- 
known solid or liquid substance, of great rigidity and 
low density. With this view, generally, also goes the 
belief that the evolution of solar heat is essentially a 
surface-action, produced, by some unexplained process, 
only where the exterior of the solar orb encounters open 
space, and not of necessity implying any great heat in 
the inner depths. The older observers, especially the 
Herschels, for the most part held theories essentially 
like that sketched above. The elder Herschel, it will 
be remembered, even contended pretty vigorously that 
the central globe of the sun is a habitable world, shel- 
tered from the blazing photosphere by a layer of cool, 
non-luminous clouds. And in more recent times Kirch- 
hoff and Zollner have maintained that the luminous 
surface is either liquid or solid. 

While it is, perhaps, not possible to demonstrate at 
present the falsity of this theory, by proving that the 
solar nucleus is neither solid nor liquid, and showing 
that the solar heat is not confined to the surface, but 
permeates the whole mass with continually increasing 
intensity near the center of the globe, it is yet evident 
enough that it meets the exigencies of the case only by 
calling in unknown and imaginary substances and op- 
erations. On the other hand, the gaseous theory, which 
is now generally adopted, involves no new kinds of mat- 
ter or unknown forces, but conceives of solar phenom- 
ena as entirely the same in kind as those we are famil- 
iar with in our laboratories, though immensely different 
in degree and intensity. 

If we only grant that the temperature rises rapidly 



SUMMARY OF FACTS, ETC. 331 

enougli from the surface downward through the solar 
globe, the whole difficulty as to the density of such a 
gaseous sphere vanishes. It is true that, on this view, 
the central temperature must be tremendous, even in 
comparison with that of the photosphere. But why 
not ? Can any reason be assigned to the contrary ? If 
we could suppose the sun wholly made of hydrogen, 
and that the ordinary relations deduced by our labora- 
tory experiments hold between the pressure and tem- 
perature through all possible ranges of both, it would 
then be a comparatively simple matter to compute the 
least central temperature which would give the solar 
globe its present density. If, however, we remember 
that other materials, and in unknown proportions, enter 
into the problem, and that in all probability our labora- 
tory-work gives only approximate formulae, it is clear 
that such a computation would be useless. We must 
content ourselves for the present with vague expres- 
sions, and say roughly that the intensity of the sun's 
internal heat must as much exceed that of the photo- 
sphere as this surpasses the mere animal warmth of a 
living body. 

But while, on the whole, it thus seems probable that 
the sun's core is gaseous, nothing could be remoter from 
the truth than to imagine that a mass of gas, under 
such conditions of temperature and pressure, would re- 
semble our air in its obvious characteristics. It would 
be denser than water ; and since, as Maxwell and others 
have shown, the viscosity of a gas increases fast with 
rising temperature, it is probable that it would resist 
motion something like a mass of pitch or putty. 

One might, then, naturally enough ask, why a sub- 
stance so widely different from gases as we know them 
by experience, and so much resembling what we are 



332 THE SUN. 

accustomed to call the semifluids, should not be classed 
with them rather than with the gases. The reply, of 
course, is, that although the substance thus bears a 
superficial likeness to the semifluids, its essential char- 
acteristics are still those of a gas, viz., continuous expan- 
sion under diminishing pressure without the formation 
of a free surface of equilibrium ; continuous expansion 
under increasing temperature without the attainment of 
a boiling-point ; and, in the case of a mixture of dif- 
ferent gases, a uniform diffusion of each, according to 
Dalton's law, without regard to specific gravity. 

Perhaps a little fuller explanation may be allowed on 
these points, which are often misunderstood. Suppose 
a mass of liquid to be contained in a close vessel, which 
it just fills, and compressed by some enormous force ; 
now let the vessel grow gradually larger, thus relieving 
the pressure. The liquid will expand, at first keeping 
the vessel full ; but at last, even if heat be supplied to 
prevent the temperature from falling, a time will come 
when the liquid will no longer fill the vessel, but an 
empty space will be left above a well-defined " free sur- 
face of equilibrium " — a space empty, that is, of the 
liquid, but of course occupied by its vapor. Now, if 
we take a similar vessel filled with a compressed gas, 
the density of which may, on account of the pressure, 
at first even exceed that of the liquid in the case just 
cited, and allow the vessel to expand in the manner de- 
scribed, at the same time supplying heat enougli to keep 
the temperature from falling, the gas will never cease 
to fill the whole vessel, nor will it ever form a free sur- 
face like the liquid, however far the enlargement of the 
vessel may be carried. 

Again, if we take a cylinder with a weighted piston, 
fitting it and moving freely in it, and, after filling the 



SUMMARY OF FACTS, ETC. 333 

space below the piston with a liquid, apply heat to it, 
we shall find that at first the temperature will rise regu- 
larly, and the liquid, expanding slightly as it warms, 
will push the piston before it. But, when a certain 
temperature, depending upon the nature of the liquid 
and the pressure exerted by the piston, has been reached, 
the liquid will cease to grow hotter by the further ap- 
plication of heat, and will begin to boil ; and the liber- 
ated vapor will raise the piston and occupy the otherwise 
vacant space above the surface of the liquid. If, how- 
ever, the space originally below the piston were occupied 
by a gas, however dense, no such thing would happen. 
The gas, on the application of heat, would rise in tem- 
perature, and expand regularly without discontinuity or 
limit. 

Finally, as to the third criterion which marks the 
difference between liquids and gases. In a mixture of 
liquids of different specific gravities, the different mate- 
rials separate and arrange themselves in strata, according 
to their weights, unless they have some chemical action 
on each other — for example, quicksilver, water, and oil. 
But a mixture of several gases, differing however widely 
in specific gravity — for example, hydrogen, oxygen, and 
carbon dioxide — behaves in no such way : under all con- 
ditions of temperature and pressure each gas distributes 
itself through the whole space, precisely as if the others 
were not present, only more slowly than if it were alone. 

Although it may not be possible, in the present state 
of science, to demonstrate that the principal portion of 
the solar mass is gaseous, this much can at least be said — 
that a globe of incandescent gas, under conditions such 
as have been intimated, would necessarily present just 
such phenomena as the sun exhibits. 

On the outer surface, exposed to the cold of space, 



334 THE SUN. 

the rapid radiation would certainly produce the con- 
densation and precipitation into luminous clouds of 
such vapors as had a boiling-point higher than that of 
the cooling surface. These clouds would float in an 
atmosphere saturated with the vapors from which 
they were formed, and also containing such other va- 
pors as were not condensed, and thus the peculiarities 
of the solar spectrum would result. On the other 
hand, the permanent gases, like hydrogen — ^those not 
subject to condensation into the liquid form under 
the solar conditions — would rise to higher elevations 
than the others, and form above the photosphere just 
such a chromosphere as we observe. Whether, from 
the mere assumption of such a constitution for the sun, 
one could work out, a priori^ the phenomena of sun- 
spots and prominences, is indeed doubtful ; but thus 
far nothing in any of them has been observed which 
appears to be inconsistent with this view of the subject 
— nothing, we say, unless it should turn out, as was 
once maintained, that the solar surface possesses, so to 
speak, "geographical" characteristics, evinced by the 
disposition to break out into sun-spots at certain fixed 
points — as if at those points there were volcanoes or 
something of the sort. Of course, the fact that the 
spots are distributed mainly in two belts parallel to the 
solar equator, involves no difficulty, for it is easy to 
conceive how, in more than one way, the sun's rotation 
might lead to such a result : but peculiarities perma- 
nently attaching to individual points on the solar sur- 
face necessarily imply rigid connections, such as are 
inconsistent with the theory of a gaseous or even of a 
fiuid nucleus. But while, as has been pointed out on 
p. 150, there is a marked tendency in spots to recur at 
or near the same points during several solar revolutions. 



SUMMARY OF FACTS, ETC. 335 

there is no evidence which establishes the existence of 
fixed spot-centers ; and the idea is to be regarded mere- 
ly as a relic of the old Herschellian theory of a solid 
sun. Still it is difficult to test the notion conclusively, 
even by means of such extended observations as those 
of Carrington or Spoerer, or the auroral periods of 
Yeeder, since the time of rotation of the solid nucleus, 
if it exists at all, is unknown, and this makes the discus- 
sion difficult and unsatisfactory. 

With reference to the constitution of the photo- 
sphere there is a general agreement among astronomers. 
A few, perhaps, still hold, as has been mentioned, to the 
idea that the visible surface is a liquid sheet, while some 
believe that it is purely gaseous ; but the whole appear- 
ance of things, the details of the granulation, the phe- 
nomena of spots and faculae, the mobility and variability 
of the floccules, all better accord with the theory adopted 
in these pages, which is a necessary consequence of the 
hypothesis that the sun is principally gaseous. It seems 
almost impossible to doubt that the photosphere is a 
shell of clouds. As to the precise constitution of this 
shell, however, the form and magnitude of the compo- 
nent cloudlets, the chemical elements involved, and the 
temperature and pressure, there is room for a good deal 
of uncertainty and diflference of opinion. The more 
common view, apparently — the one, certainly, which the 
writer has hitherto held — is, that the clouds are formed 
mainly by the condensation of the substances which are 
most conspicuous in the solar spectrum, such as iron 
and the other metals. As to the form of the clouds, 
also, it has usually been assumed that, as a consequence 
of the ascending currents by whicli they are formed, 
they are columnar, their height being much greater 
than their other dimensions. 



336 THE SUN. 

Professor Hastings has proposed a somewhat differ- 
ent theory (already referred to on page 282), which 
avoids some of the difficulties of the received doctrine, 
though not without encountering others which seem 
just as formidable. 

One main peculiarity is the assumption that the 
photospheric " clouds " are formed by the precipitation 
of either carbon, silicon, or boron (the three members 
of the carbon group), to the exclusion of other sub- 
stances which are less refractory (have lower hoiling- 
points)^ and therefore escape precipitation. Those 
bodies which have boiling-points higher than that of 
this photosphere-element, as it may be called, will, there- 
fore, not exist to any extent in the vaporous atmosphere, 
having suffered precipitation before they reach the visi- 
ble surface. Those only will show their lines in the 
spectrum which have lower boiling-points, and so do 
not suffer precipitation at the temperature of the photo- 
sphere. He gave this as the reason why the lines of 
silicon, etc., do not appear in the solar spectrum, a re- 
mark which has now lost its force, since the later work 
of Rowland and others shows that the lines of carbon 
and silicon are really present. The carbon lines are 
several hundred in number, but not conspicuous because 
they fall in the violet and ultra-violet. The lines of sili- 
con are strong, but not numerous. It will at once be seen 
that, if this view is true, the temperature of the photo- 
sphere is that of the boiling-point (under the local con- 
ditions of pressure) of the silicon or carbon, or what- 
ever it is which forms the clouds. As an objection to 
the view, it immediately occurs to one that, if the car- 
bon, for instance, is precipitated at and below some 
special elevation, yet tlie vapors of iron, sodium, and all 
the other solar metals will rise above it, and, in their 



SUMMARY OF FACTS, ETC. 337 

turn, will find a level and temperature of precipitation ; 
so that the photospheric clouds, instead of being com- 
posed of any single substance, would contain all which 
can find a level and temperature of precipitation any- 
where in the solar atmosphere. As to the form of the 
fioccules, it would seem that the successive precipitation, 
at different levels and temperatures of different ele- 
ments in an ascending current, must result in clouds of 
great vertical extent — '^ columnar," as we have called 
them. Professor Hastings rather demurs to this, how- 
ever, saying that in his observations he has met '' noth- 
ing which would indicate a columnar form of the gran- 
ules under ordinary circumstances." 

As regards the explanation of the absorption layer 
which darkens the edge of the sun, and the theory of 
sun-spots and their penumbra, we give his own words : 

*' The precipitated material rapidly cools on account of its 
great radiating power, and forms a fog or smoke which settles 
slowly through the spaces between the granules, until revolatilized 
below. It is this smoke which produces the general absorption 
at the limb, and the ' rice-grain ' structure of the photosphere. 

''Where any disturbance tends to increase a downward con- 
vection current, there is a rush of vapors at the outer surface of 
the photosphere toward this point. These horizontal currents or 
winds carry with them the cooled products of precipitation, which, 
accumulating above, dissolve slowly below in sinking. This body 
of smoke forms the solar spot. 

"The upward convection currents in the region of the spots 
are bent horizontally by the centripetal winds. Yielding their 
heat now, by the relatively slow process of radiation, the loci of 
precipitation are much elongated, thus giving the region imme- 
diately surrounding a spot the characteristic radial structure of 
the penumbra. 

"This conception of the nature of the penumbra implies a 
ready interpretation of a remarkable phenomenon, amply attested 
by the most skillful observers, and, as far as my knowledge goes, 



338 THE SUN. 

wholly unexplained, namely, the brightening of the inner edge of 
the penumbra in every well-developed spot. 

^' This interpretation is, perhaps, most readily imparted by a 
comparison of the hot convection currents in the two cases. 
When the convection current is rising vertically, the medium is 
cooled by expansion until the precipitation temperature is reached, 
when all the coudensible material appears suddenly, save as it is 
somewhat retarded by the heat liberated in the act. Immediately 
afterward the particles become relatively dark by radiation. In 
the horizontal currents a very different condition of things ob- 
tains. Here the medium does not cool dynamically, by expansion, 
but only by radiation ; hence (since the radiation of the solid par- 
ticles is enormously greater than that of the supporting gas) prac- 
tically by that of the particles themselves. Thus, after the first 
particle appears, it must remain at its brightest incandescence 
until all the material of which it is composed is precipitated. 
From this we see that such an horizontal current must increase 
gradually in brilliancy to its maximum, and then suddenly dimin- 
ish — an exact accordance with the facts as observed." 

The idea tliat the stratum which produces the general 
absorption at the limb of the sun is a veil of " smoke " 
— i. e., of the same minute particles which constitute 
the photosphere, but cooled to relative darkness — has 
been already alluded to in a preceding chapter. So far 
as we know, it is novel and valuable, clearing up a good 
many embarrassing difficulties. It is so obvious, on 
reflection, that something of the sort must accompany 
the photosphere, that it is surprising that the idea had 
not been thought of before. Of course, the particles 
formed by condensation must, many of them at least, 
be carried by the ascending currents high above the 
point of their formation, and cooled so much as to be- 
come relatively dark in comparison with the more vivid 
incandescence of the regions below, just as the ascend- 
ing particles of carbon, unconsumed and cooled, consti- 
tute the smoke of a fire. 



SUMMARY OF FACTS, ETC. 339 

As regards the explanation of spot phenomena, we 
see no special advantage in the idea proposed. The 
received theory regards the general brightening at the 
inner edge of the penumbra as produced by the con- 
vergence of the luminous filaments, rendered horizontal 
by the indraught. The qicasi-hnlhous termination of 
the filaments occurs only occasionally, and is probably 
merely an illusion due to "irradiation." As already 
stated on page 126, with a large telescope, and under 
the finest optical conditions, these " bulbs " assume the 
appearance of " fish-hooks," so to speak, of extreme 
brilliance. And it is difficult, though perhaps not im- 
possible, to reconcile the smoke theory of sun-spot dark- 
ening with the observations of the writer and Duner 
upon the sun-spot spectrum (page 132), which seem to 
show that the absorbing medium is mainly gaseous. It 
may be that such a smoke as the theory supposes would 
carry with it a sufficient quantity of cooled vapors to 
explain the spindle-shaped, close-packed dark lines that 
are observed, while the bright lines, here and there ob- 
served, can be accounted for as due to overlying gases. 

The idea that carbon may be the main constituent 
of the photosphere is by no means new : it was first 
seriously advanced, we believe, by Johnstone Stoney, of 
Dublin, as early as 1867, mainly on physico-chemical 
grounds, and is enthusiastically advocated by Sir Robert 
Ball in his recent " Story of the Sun." It is quite pos- 
sible that the objection based upon the lower condensa- 
tion temperature of iron and other metallic vapors may 
be fairly met by such considerations as explain the pres- 
ence of a certain amount of water- vapor above the clouds 
in our own atmosphere. 

As regards the " reversing stratum " very little need 
be added. Mr. Lockyer indeed denies its existence— 



340 THE SUN. 

that is, in the sense tliat there is a thin stratum, close 
above the surface of the photosphere, in which most of 
the dark lines of the solar spectrum originate. He 
maintains, on the contrary, in accordance with his " dis- 
sociation theory," that certain of the lines, due to sub- 
stances the most nearly elementary, and having their 
molecules in the highest stage of dissociation, originate 
only deep down in the solar atmosphere where the heat 
is most intense ; others, due to vapors with molecules 
somewhat less simple, have their birth a little higher ; 
and others yet, due to molecules the most complex, are 
produced only in the most elevated regions of the solar 
atmosphere; each elevation thus being responsible for 
its own special family of spectrum lines. 

If, however, we reject this theory as " not proven," 
we get results not very different. 

The vapors of the photosphere and chromosphere 
are not to be thought of as entirely separate and dis- 
tinct. All the gases are found together in the inter- 
stices between the cloud-granules of the photosphere — 
the unknown substance which produces the green line 
in the spectrum of the corona, the hydrogen, the cal- 
cium, and helium w^hich characterize the chromo- 
sphere, and the metallic vapors which give tlie revers- 
ing layer its peculiar properties — these all exist together 
in the lower depths, unless, indeed, it may possibly be 
the case that at the greater elevations some compound 
bodies are formed which can not exist in the fiercer 
fires below. So far as we can distinguish between 
these different portions, we may define the photosphere 
as the shell within which precipitation is taking place ; 
the reversing layer, as that lowest region of the solar 
atmosphere which contains sensibly all the gases indi- 
cated by the spectroscope ; the chromosphere, as the 



I 



SUMMARY OF FACTS, ETC. 341 

region of hydrogen, calcium, and helium ; and the 
corona, as that upper domain of the solar atmosphere 
which becomes observable only during solar eclipses. 
But the coronal gas itself is most conspicuous and 
abundant right in the photosphere and reversing layer, 
and the same is true of the hydrogen of the promi- 
nences. 

It is well, also, to bear in mind that, if any sub- 
stances decomposable by heat exist upon the sun at all, 
we must expect to find them in the higher and cooler 
regions of the solar atmosphere. In and near the pho- 
tosphere, or underneath it, matter must be in its most 
elemental state. 

As to the mechanism of the chromosphere and 
prominences, if we may use the expression, much cer- 
tainly remains to be learned. In many cases, indeed, 
perhaps in most, the forms and behavior of the protu- 
berances are satisfactorily enough accounted for by sup- 
posing that the heated hydrogen and its associate vapors 
is simply forced up into cooler regions by pressure from 
below — a pressure which must result from the down- 
ward movement of the great mass of precipitated mat- 
ter which forms the photosphere. But evidently this 
is not the whole story. We must have recourse to ideas 
of a diflferent order to account for the somewhat rare, 
but still really numerous and well-authenticated in- 
stances when the summits of prominences have been 
seen to rise in a few minutes to elevations of two or 
three hundred thousand miles, the upward motion being 
almost visible to the eye at the rate of a hundred miles 
a second or more. 

Very perplexing, also, is the indubitable fact that 
clouds of this prominence-matter sometimes gather and 
form without any apparent connection with th^ chromo- 



342 THE SUN. 

sphere below, apparently just as clouds form in our own 
atmosphere, by the condensation of vapor, before invis- 
ible. On the whole, it looks very much as if we must 
regard the prominences as differing from the surround- 
ing medium mainly, if not wholly, in their luminosity 
—as simply superheated portions of an immense atmos- 
phere. 

But, then, we immediately encounter the difficulties 
so ably urged by Lane, Lockyer, and others, that the 
existence of hydrogen of any appreciable density, at the 
elevation of even a hundred thousand miles, implies a 
density and pressure at the surface of the photosphere 
so high as to be entirely inconsistent with the spectro- 
scopic phenomena there manifested — unless, indeed, 
under solar conditions, the action of gravity upon the 
gases of the solar atmosphere is modified by some re- 
pulsive force. That such a force is at least conceivable, 
is obvious from tlie behavior of the tails of comets; 
and many features in the corona point in the same di- 
rection. Of its nature and origin we can not, however, 
assert anything as yet. 

Even more difficult than the problem of the chromo- 
sphere is that of the corona. While it is something to 
know that the phenomenon is mainly solar, and that, 
therefore, it must rank in magnitude and importance 
with the most magnificent of natural objects, we have 
yet to find a satisfactory explanation of many of its 
most obvious features. It is certainly very complex — 
matter meteoric and matter truly solar ; orbital motion, 
solar attraction, atmospheric resistance, and actions 
thermal, electrical, and magnetic, are probably all com- 
bined. 

At present it would seem that the most important 
and fundamental problems of solar physics which are 



SUMMARY OF FACTS, ETC. 343 

now pressing for solution are these : first, a satisfactory 
explanation of the peculiar law of rotation of the sun's 
surface ; second, an explanation of the periodicity of 
the spots, and their distribution ; third, a determination 
of the variations in the amount of the solar radiation 
at different times and different points upon its surface ; 
fourth, a satisfactory explanation of the relations of the 
gases and other matters above the photosphere to the 
sun itself — the problem of the corona and the promi- 
nences ; and, fifth, the discovery of some reasonable 
hypothesis as to the sun's loss of heat by radiation 
which would reconcile our estimates of its age and 
probable future endurance with the demands of evolu- 
tionary theories of the planetary and stellar systems. 

One might name many others of hardly less interest, 
such as that which has to do with the intimate connec- 
tion between terrestrial magnetism and the condition 
of the solar surface ; but, on the whole, the five named 
seem to be those the solution of which would most ad- 
vance our science. Not, of course, that we are to sup- 
pose that even their solution would bring us in sight of 
the end or limit of knowledge. Each onward step only 
opens before us a new, wider, and more magnificent 
horizon, with infinity still beyond it. 



SUPPLEMENTARY NOTE. 



HELIUM, ITS IDENTIFICATION AND PROPERTIES. 

While tins work was under revision and passing 
through the press fresh announcements respecting heli- 
um followed each other so rapidly that it soon became 
evident that it would be useless to attempt to include 
them all in the text, and that the better course would be 
to add a supplementary note which should represent as 
nearly as possible the state of our knowledge at the 
date of final publication. 

The famous D3 line was first seen in 1868, when the 
spectroscope was for the first time directed upon a solar 
eclipse. Most of the observers supposed it to be the D 
line of sodium, but Janssen noted its non-coincidence ; 
and very soon, when Lockyer and Frankland took up 
the study of the chromosphere spectrum, they found 
that the line could not be ascribed to hydrogen or to 
any then known terrestrial element. As a matter of 
convenient reference Frankland proposed for the un- 
known substance the provisional name of '^ helium" 
(from the Greek " helios," the sun), and this ultimately, 
though rather slowly, gained universal acceptance. 

Within a year two other lines (X 7,065 and \ 4,472) 
were discovered in the chromosphere spectrum by Eayet 
and Respighi respectively, which like D3 are always 



I 



supplemeinTahy note. 345 

present in the prominences, but have no corresponding 
dark lines in the ordinary solar spectrum. It was of 
course early suggested, but without proof, that these 
lines also were due to helium. Since then some eisrht 
or ten other lines have been found, frequently, but not 
always, presenting themselves in the chromosphere 
spectrum, and, like the first three, also without dark 
analogues. Moreover, still more recently, Dg and its 
congeners have been detected in stellar spectra — darl^ 
in the spectra of the " Orion stars," hright in the spectra 
of certain variables and of the so-called Wolf-Eayet 
stars; and hoth hright and darh in yS Lyrse and the 
"new star" of Auriga which appeared in 1892. 

Naturally there has been much earnest searching 
after the hypothetical element, but until very recently 
wholly without success ; though it should be mentioned 
that in 1881, Palmieri, the director of the earthquake 
observatory upon Vesuvius, announced that he had 
found Dg in the spectrum of one of the lava minerals 
with which he was dealing. But he did not follow up 
the announcement with any evidence, nor has it ever 
received any confirmation, and from what we now know 
as to the conditions necessary to bring out the helium 
spectrum, there is every reason to suppose that he was 
mistaken. 

The matter remained a mystery until April, 1895, 
when Dr. Ramsay, who was Lord Rayleigh's chemical 
collaborator in the discovery of aro;on, in examining the 
gas liberated by heating a specimen of Norwegian 
cleveite, found in its spectrum the Dg line, conspicuous 
and indubitable. The mineral was obtained from Hille- 
brand, one of our American chemists, who had pre- 
viously studied it, and ascertained that it could be made 
to give off a gas which he identified with nitrogen. It 



346 SUPPLEMENTARY NOTE. 

really was nitrogen in part, but Ramsay suspected that 
he should also find argon^ as he did — and helium be- 
sides, which was unexpected. 

Cleveite is a species of uraninite or pitch blende, and 
it soon appeared that helium could be obtained from 
nearly all the uranium minerals, and from many others ; 
from many, mingled with argon ; from others, nearly 
pure. In fact, it turns out to be very widely distributed, 
though only in extremely small quantities, and generally 
'' occluded," or else in combination — seldom, if ever, free. 
It has been detected in meteoric iron, in the waters of 
certain mineral springs in the Black Forest and Pyre- 
nees, and Kayser even reports traces of it in the atmos- 
phere at Bonn. 

It is generally obtained by heating the substance that 
contains it in a close vessel connected with an air-pump 
of some kind by w^hicli the liberated gases are drawn off 
and collected. They are then laboriously treated to 
remove as far as possible all the foreign elements (nitro- 
gen, etc.), since the presence of as much as five or ten 
per cent, of any other gas prevents the new elements 
from giving any spectroscopic evidence of their pres- 
ence ; they are too shy and modest to obtrude them- 
selves. In many cases, as has been said, argon and 
helium come off together, and certain lines in their 
spectrum are nearly coincident, so that for a time there 
was supposed to be some close bond of connection be- 
tween them. The latest observations, however, make 
it certain that this is not so : as Mr. Lockyer puts it, 
'' argon is of the earth, earthy, but helium is distinctly 
celestial." 

Its spectrum has been thoroughly studied by Crookes, 
Lockyer, and Runge, w^ho agree as to all its leading 
characteristics. 



SUPPLEMENTARY NOTE. 347 

Range, whose work is most complete and authorita- 
tive, finds that its lines have a remarkably regular ar- 
rangement, falling into two distinct ''sets," each set 
consisting of a principal series and two subordinate 
ones, the lines in each series corresponding very accu- 
rately to a formula quite similar to that discovered by 
Balmer as governing the hydrogen spectrum. 

In the whole spectrum he finds (by photography 
mainly) sixty-seven lines, twenty of which only are in 
the visible part of the spectrum. Of the sixty-seven, 
twenty-nine belong to the first " set " and thirty-eight 
to the second. Of the twenty " visual " lines, thirteen 
have been observed in the spectrum of the chromo- 
sphere ; the missing lines all belong to the second subor- 
dinate series of the first " set,"' and are so faint in the 
artificial spectrum of the gas that their failure to be 
found in the chromosphere needs no explanation. 

The fact that the lines thus divide into two mathe- 
matically independent " sets " has led Runge to believe 
that the helium obtained from the minerals is really a 
mixture of two distinct gases, and he has found it pos- 
sible to partially separate the two by a process of diffu- 
sion. The true helium, the one that gives D3 and the 
other lines that are always present in the chromosphere 
spectrum, he considers to be the denser of the two ; the 
spectrum of the other contains most of the lines that 
appear only occasionally in prominences. With this 
view Mr. Lockyer is entirely in agreement. The lighter 
component has as j^et received no name. Lockyer calls 
it simply X. 

The lines of the series to which D3 belongs are all 
double^ having a very faint companion on the lower 
(i. e., red- ward) side, extremely close to the principal 
line. When Runge announced this discovery early in 



348 SUPPLEMENTARY NOTE. 

June it at first produced sometliing like consternation 
among spectroscopists, for at that time there still re- 
mained more or less doubt as to the validity of Ram- 
say's identification, and the solar Dg had never been 
observed to have such a companion. Very soon, how- 
ever, Hale, Huggins, Reed, and other observers who 
had sufiiciently powerful instruments, detected the lit- 
tle attendant of Dg in the spectrum of prominences, so 
that the momentary distrust was replaced by absolute 
confidence. 

As to the physical and chemical properties of the 
new gas our knowledge is still limited and our conclu- 
sions embarrassed by the uncertainty whether we are 
dealing with a single element or a mixture — whether 
Dr. Ramsay has introduced to the world one infant or 
a pair of twins. 

The gas liberated from cleveite, and purified as far 
as possible, shows a density just a little more than double 
that of hydrogen, and is therefore much lighter than 
any other known gas except hydrogen itself. If it is a 
mixture the lighter gas must have a density less than 
two, and may even prove to be lighter than hydrogen, 
while the true Dg helium may have a density anywhere 
between two and four, depending on the proportions of 1 
the mixture and the density of the lighter compound. 
It would be very fine, we may remark in passing, if 
the lighter component could have been identified with 
" coronium," but this seems impossible since the char- 
acteristic 1,474 line (\ 5,316) does not appear at all in 
the spectrum of terrestrial " helium " derived from any 
source. 

Ramsay's acoustic experiments tend to show that 
helium, like argon, is monatomic, but can hardly be 
considered conclusive. If he is right, the atomic weight 






SUPPLEMENTARY NOTE. 349 

of helium must be not far from four ; but thus far all 
attempts to make it enter into chemical combination 
have failed, though it seems rather probable that in the 
uraninite minerals it is held by stronger bonds than 
those of mere occlusion. 

Olszewski has tried his best to liquefy the gas, but 
thus far unsuccessfully ; the methods that have con- 
quered every other gas, hydrogen itself included, have 
failed with helium — a circumstance very remarkable, 
since generally a denser gas liquefies more easily than a 
lighter one, and hitherto hydrogen has stood pre-emi- 
nent in its refractoriness. 

Probably the question has suggested itself to every 
reader how it happens that helium, so conspicuous in 
the atmosphere of the sun and many stars, should be so 
nearly absent from our own atmosphere and so scantily 
present in any form upon the earth. The answer seems 
to depend upon two facts — the chemical inertness of 
the substance and its low density. 

According to Johnstone Stoney's deductions from 
the accepted theory of gases, wo free gas of low density 
can remain permanently upon a heavenly body of small 
mass and habitable temperature, but the molecules will 
fly off into space. A particle leaving the earth with a 
velocity of about seven miles a second would never re- 
turn to it. Now, according to the dynamic theory of 
gases, the molecules of our atmosphere are flying swiftly 
about with velocities (at ordinary temperatures) of from 
1,500 to 10,000 feet per second ; the heavier molecules, 
like those of oxygen and nitrogen, move comparatively 
slowly, but if any free hydrogen or helium is present 
its molecules take up velocities several times more swift, 
and any that may happen to be near the upper limits of 
the atmosphere w^ould be likely to be thrown off into 



350 SUPPLEMENTARY NOTE. 

space. Ill tlie case of the moon even the oxygen and 
the nitrogen would go, since she is so small that a ve- 
locity not much exceeding a mile a second would carry 
them off. If this is correct it is easy to see w^hy we 
now have no appreciable quantity of free hydrogen or 
other light gas in our atmosphere. 

But while we have no atmospheric hydrogen to 
speak of, hydrogen in combination is extremely abun- 
dant ; one eighth part by weight of all the water in the 
sea is hydrogen ; and hydrogen combines freely with 
many other elements besides oxygen, so that we contin- 
ually liberate it in all sorts of chemical decompositions. 
Helium, on the other hand, enters into combination 
most sparingly, is therefore scarce, and even when pres- 
ent is, as we have said before, not easy to detect. 

November. 1895, 



INDEX. 



ABBE, extent of corona in eclipse 
of 1878, 246. 

Absorption of sun's rays by the at- 
mosphere of the earth, 297, 298. 

Actinic or chemical rays, 285. 

Adjustment of focal plane of tele- 
scope to the slit of the spectro- 
scope for observations upon the 
spectrum of the chromosphere, 
. 207. 

slit of spectroscope for ob- 
servations of the prominences, 
210, 211. 

Age and duration of the sun, 316- 
318. 

Airy, solar parallax from the transit 
of 1874, 24. 

Allotropic states of chemical ele- 
ments, 91. 

American method of photographing 
the transit of Venus, 28, 29. 

Analyzing spectroscope, 73. 

Andrews, critical temperature of a 
gas, 329. 

Angstrom, A. J., early studies in 
spectrum analysis, 60, 80. 

— map of solar spectrum, 78, 92, 93. 

Angstrom, K., measures of solar 
radiation, 304. 

Animal, body of, regarded as a ma- 
chine, 3, 4. 
: Arago, diminution of brightness at 
the limb of the sun, 279. 

Aristarchus, method of determining 
the sun's parallax, 14. 



" Arrow-head " appearance of hy- 
drogen lines in chromosphere 
spectrum, 209, 210. 
I Ascension Island, 19. 

Asteroids, observed for solar paral- 
lax, 21. 

Aurora boreal is, its spectrum not 
to be identified with that of the 
corona, 260. 

relation to sun-spots, 164, 165. 

resemblance between its 

streamers and those of the co- 
rona, 265, 273. 

Axis of the sun, 144-146. 

table giving its posi- 
tion-angle for difterent times of 
the year, 146. 

BALL, Sir EGBERT, on carbon 
in sun, 339. 
Basic lines in solar spectrum, 92, 93. 
Barker, dark lines in spectrum ot 
the corona, 261. 
' Belli, photometric observation upon 
I the brightness of the corona, 254. 
i Belopolsky, rotation of sun, 143. 

Bessemer converter, compared with 
\ the solar radiation, 279, 324. 
I Biela, brightness of the inner corona, 
255. 
Bigelow, F. II., on sun's rotation 
period, 137. 

theory of corona, 273. 

Blueness of sunlight before suffer- 
ing atmospheric absorption, 284. 



352 



IXDEX. 



Bolometer described, 299. 

— determination of true value of 
the solar constant, 298. 

— sensitiveness of, 299. 
Bouguer, measurement of the sun's 

light, 275. 

Brester, theory of prominences, 228. 

Brightness of the corona, 253-256. 

Bullock, drawing of eclipse of 1868, 
244. 

Bunsen, arrangement of spectro- 
scope scale, 63. 

— work upon the solar spectrum in 
connection w^ith Kirchhoff, 60. 

Burning-glass, efiect of, 308, 309. 

(1 LITsE, in chromosphere and 
J prominences, 197, 206, 209, 210, 
232. 

photograph of prominence, 232. 

photograph of its double re- 
versal, 209. 

Candle-power, or photometric unit, 
defined, 275. 

Calcium-light compared with sun- 
light, 278, 324. 

Calcium lines in spectrum, see H and 
K lines. 

Calory, or thermal unit, defined, 289. 

Capocci, theory that spots are due 
to volcanic eruptions on the sun, 
178. 

Carbon, as chief constituent of pho- 
tosphere, 339. 

Carrington, discovery of sun's equa- 
torial acceleration and formula 
for it, 138-140. 

— distribution of sun-spots, 147,148. 

— method of determining the posi- 
tion of a spot on the sun, 45. 

— motion of spots in latitude, 146. 

— observation of remarkable solar 
outburst, November 1, 1859, 120. 

— period of sun's rotation, 137, 138. 

— position of sun's axis, 144. 
Cassini, observations for solar par- 
allax, 18. 



Christie, solar eyepiece, 58. 
Chromosphere defined, 7, 192. 
Cleveite, mineral from which helium 

is obtained, 345, 346-348. 
Coal, consumption of which would 

be required to keep up the solar 

radiation, 289. 
Comets' tails, their analogies to the 

streamers of the corona, 265, 342. 
Comparison-prism, 84. 
Concave-grating spectroscope, 70. 
Condensation theory of solar heat, 

314-316. 
Constancy of solar heat during the 

historic period, 311. 
Constitution of sun, 8, 325-335. 
Contact observations at the transit 

of Venus, 23, 24. 

by means of photography, 26. 

Corona, brightness of, 253-256. 

— defined, 7. 

— examined by slitless spectroscope, 
261, 262. 

Corona-line in the spectrum, discov- 
ery, 250. 

duplicity of, 257. 

jnap, 258. 

not identical with line in spec- 
trum of aurora borealis, 260. 

Cornu, solar photography, 47. 

Crew, IL, spectroscopic determina- 
tion of the sun's rotation period, 
100. 

Critical temperature of a gas, 329. 

Croll, hypothesis that a portion of 
the sun's energy may have origi- 
nated in a collision with a star, 
318. 

Crookes, spectrum of helium, 346. 

Crova, pyrheliometer, 292. 

— value of solar constant, 296. 
Cyclonic motion in sun-spots, 126, 

183,184. 



D 



LINE of helium, 88, 198, 
3 220, 345-348. 

dark in sun-spot spectrum, 135. 



INDEX. 



353 



D lines of sodium, doubly reversed, 
208. 

Dalton, law of gaseous mixture, 332, 
333. 

Dark lines in the solar spectrum dis- 
covered, 59. 

explanation of, 80, 81. 

in spectrum of the corona, 261. 

Dartmouth College spectroscope, 73, 
74. 

Davis, photograph of eclipse of 1871, 
248. 

Dawes, "holes" in nucleus of sun- 
spot, 118. 

: — solar eyepiece, 58. 

Dehra Dun, photographs of sun, 52. 

De La Eue, the Kew photohelio- 
graph, 48, 49. 

photographs of the eclipse 

of 1860, 195. 

measures of sun-spot pe- 

— numbra, 130. 

5 planetary influence on sun- 
spot development, 158. 

relation of Wolfs "relative 

numbers*' to the spotted area of 
the sun, 154. 

Denza, bright lines in corona spec- 
trum, 260. 

Derham, volcanic theory of sun- 
spots, 178. 

Deslandres, on spectrum of faculse, 

. 109. 

spectro-heliographic work, 236. 

Detached cloud-formed prominences 
and their development, 221-222. 

Development of sun-spots, 122. 

Deville, estimate of the temperature 
of the sua, 305. 

Diameter and dimensions of the sun, 
38, 323. 

illustrations, 39. 

Diffraction grating, 66, 70. 

— spectroscope, 68-70. 

— spectrum, 72. 

Dimensions of sun-spots, 127, 128. 
Diminution of brightness at limb of 
24 



the sun, 45, 108, 279-284, 324, 
337. 

Discovery of bright line in corona 
spectrum, 250. 

dark lines in solar spectrum, 59. 

dark lines in spectrum of co- 
rona, 262. 

elements present in the sun, 

86-88. 

equatorial acceleration of the 

sun, 138-140. 

explanation of cause of dark 

lines, 80. 

gaseous constitution of the 

prominences, 197. 

magnetic relations of sun- 
spots, 162. 

periodicity of sun-spots, 151. 

reversing layer of the sun, 82. 

spectroscopic method of ob- 
serving prominences, 198. 

sun-spots, 114. 

terrestrial helium, 345-348. 

Displacement and distortion of lines 
by motion, 97-101, 208. 

Dissolution and disappearance of 
sun-spots, 122-124. 

Distances (relative) of planets, 16. 

Distance of the sun from the earth, 
illustrations, 36, 37. 

Distortion of forms of prominences, 
by spectroscope, 205. 

Distribution of sun-spots and prom- 
inences in solar latitude, 147, 148, 
214. 

Don Ulloa, observation of "hole in 
the moon," in the eclipse of 1778, 
194. 

Dopplers principle, 97-98. 

Draper, Dr. Henry, oxygen in the 
sun, 94, 95. 

Draper, J. W., early spectroscopic 
researches, 60. 

Drawings of corona, discrepancies, 
239. 

Dulong and Petit, law of radiation, 
306, heats, 92. . 



354 



INDEX. 



Duner, spectroscopic determination 

of the sun's rotation period, 101. 
— ■ — on sun-spot spectrum, 132, 339. 
Duration of sun-spots, 119. 

II^AETH, dimensions of the, 12. 
^ — her share of the solar radi- 
ation, 290. 

Eastman, photometric observations 
during eclipse of 1869, 253. 

Eclipse, solar, 1706—194; 1715— 
194; 1733—193; 1778—194; 1806 
—194; 1842—194, 254; 1851—195 
1857—240 ; 1860—195, 241, 242, 256 
1867—243; 1868—196, 197, 244 
1869—245, 250, 260 ; 1870—83 ; 1871 
—246-248, 256, 265 ; 1878—239, 249, 
264; 1882—251 ; 1889—253 ; 1893— 
255. 

general phenomena, 237, 238. 

Ecliptic, defined, 6. 

Effect of changes in solar atmos- 
phere upon terrestrial conditions, 
303. 

Effective temperature of the sun, 
305. 

Egoroff, identities A and B lines of 
solar spectrum as due to oxygen, 
93. 

Electric light compared with sun- 
light, 278, 324. 

Elements known to be present in 
the sun, table, 87, 88. 

Encke, discussion of transits of 
Venus in 1761 and 1769, 22, 36. 

Energy (total) of solar radiation, 
288-290. 

Energy, terrestrial, mainly derived 
from solar heat, 2, 3. 

other sources than solar heat, 4. 

Equatorial acceleration of the sun, 
138-140. 

explanation suggested, 141-144. 

Ericsson, estimate of the sun's tem- 
perature, 305. 

— experiment upon radiation of 
molten iron, 309. 



Ericsson, measure of solar heat, 293. 

— solar engine, 290. 

Eruptive prominences, 218-223. 

Experiment, showing that the black- 
ness of the dark lines in the spec- 
trum is only relative, 80, 81. 

Explanation of the sun's eruptive 
action caused by the constriction 
of the photosphere, 227. 

FABEICIUS, discovery of sun- 
spots, 114. 

Faculse, 106-108. 

spectrum of, 109. 

identical or not with promi- 
nences, 109. 

used in determining sun's ro- 
tation period, 140. 

Faye, explanation of the sun's equa- 
torial acceleration, 143. 

— formula for the acceleration, 139. 

— theories of sun-spots, 181, 182- 
185. 

— computation of solar parallax, 
23. 

Ferrers, observation of eclipse of 
1806, 192. 

Fizeau, comparison of electric and 
calcium light with sunlight, 278. 

Flamsteed, method of deducing so- 
lar parallax from observations of 
Mars, 15, 19. 

Flashes reported by Peters in sun- 
spots, 123. 

Fa3nander, drawing of eclipse of 
1871, 247. 

Forbes, value of solar constant, 296. 

Foucault, comparison of electric and 
calcium lights with sunlight, 278. 

— determination of the velocity of 
light, 34. 

— early spectroscopic researches, 60. 
Fourteen hundred and seventy-four 

line, 257-260, 262. 
Frankland names helium^ 344. 
Fraunhofer, discovery of dark lines 

in solar spectrum, 59. 



INDEX. 



355 



Fraunliofer, coincidence of D line in 
solar spectrum with bright line in 
flame spectrum, 80. 

— map of the spectrum, 77. 

— use of a prism to separate overly- 
ing diffraction spectra, 68. 

Frost, E. B., temperature of a sun- 
spot, 170. 

— solar radiation, 302. 

GALILEO, discovery of sun-spots, 
114. 

— theory of sun-spots, 178. 

Gas, Dalton's law of mixture, 332, 
333. 

— distinctive properties, 332, 333. 

— Lane's law of temperature and 
condensation, 315. 

— liquefaction and critical tempera- 
ture, 329. 

— viscosity at high temperatures, 
331. 

Gaseous condition of the sun's nu- 
cleus, 327-332. 

Gautier, relation between magnet- 
ism and sun-spots, 163. 

Gill, observations of Mars for deter- 
mination of solar parallax, 19-21. 

— observations of asteroids, 21. 
Gilman, corona of eclipse of 1869, 

244. 

Gould, diminution of the earth's 
temperature at sun-spot maximum, 
172. 

Granulation of the sun's surface, 
102-105. 

Grant, early recognition of the chro- 
mosphere, 195, 200. 

Grating, diffraction, used in spectro- 
scope, 66-71, 204. 

concave, 70. 

Gray, Wilson and, on temperature 
of sun, 307. 

Greenwich magnetic record for Au- 
gust 3 and 5, 1872, 168. 

— solar photographs, 52, 
Greo^orv first calls attention to tran- 



sits of Venus as a means of deter- 
mining the sun's parallax, 22. 
Grosch, drawing of eclipse of 1867, 
243. 

HLINE in spectrum of corona, 
258, 260. 

in spectrum of faculse, 109. 

reversal in prominences, 206, 

231-233. 

double reversal, 109, 231. 

reversed in the spectrum of 

sun-spots, 109, 134. 
Habitability of the sun, 179, 330. 
Hale, G. E., on spectrum of faculae, 

109. 

— — spectro-heliographic work, 
234, 235. 

— recognition of duplicity of Dg in 
chromosphere spectruni, 348. 

Halley, determination of the sun's 
parallax by transit of Venus, 22. 

Hansen, detection of error in the re- 
ceived value of the sun's parallax, 
23, 36. 

Harkness, observation of the bright 
line in the corona spectrum, 250. 

Hastings, comparison of the spec- 
trum of the sun's limb with that 
of the central portion of the disk, 
83. 

— smoke-like nature of the layer 
which causes the darkening of the 
sun's limb, 282, 337. 

-- theory of the constitution of the 
sun, 336-340. 

Heat derived from stars and meteors, 
4. 

Heliometer described, 19. 

Helioscopes and helioscopic eye- 
pieces, 54-57. 

Helium and its characteristic lines, 
88, 260. 

— identification as a terrestrial ele- 
ment by Eamsay, 88, 345. 

— in the spectrum of the promi- 
nences, 206, 207, 220, 348. 



356 



INDEX. 



Helium, note on its discovery and 
properties, 344-350. 

Helmholtz, contraction theory of the 
solar heat, 315. 

Henry, observations with the ther- 
mopile upon radiation of sun-spots 
and different portions of the sun's 
disk, 170, 298. 

Herschel, Sir John, measurement of 
the sun's heat, 286-289. 

— meteors as the cause of the 

sun's equatorial acceleration, 141. 

solar eyepiece, 55. 

theory of sun-spots, 179. 

Herschel, Capt. John, observation of 
prominence spectrum in 1868, 197. 

Herschel, Sir W., relation between 
sun-spots and price of wheat, 152. 

— theory of the sun-spots and the 
sun's constitution, 179. 

Higgs, G., photographic maps of 
solar spectrum, 79. 

Hillebrand, first discovers gas in 
cleveite, 345. 

Hodgson, observation of solar out- 
burst in 1859, 120. 

Horrebow, anticipation of the peri- 
odicity of sun-spots, 152. 

Howlett, views on nature of sun- 
spots, 129. 

Huggins, granulation of the sun's 
surface, 105. 

— use of widened slit in observing 
forms of prominences, 201. 

Hydrogen-lines in the spectrum of 
the corona, 258. 

ICE, quantity which would be 
melted in a minute by the sun's 
radiation, 288, 289. 
Intra-Mercurial planet, 314. 
Investigation as to the influence of 
the planets upon the generation of 
sun-spots, 158, 159. 

JANSSEN, discovery of method of 
observing prominences by means 
of the spectroscope, 196, 197. 



Janssen, medal from French Gov- 
ernment, 200. 

— observation of the eclipse of 1868, 
198, 344. 

— observations of the eclipse of 1871 
and recognition of bright lines ot 
hydrogen and dark Eraunhofer 
lines in the corona spectrum, 258. 

— observation of Venus on the co- 
rona, 229. 

— photographic contact at the tran- 
sit of Venus, 26. 

— Reseau Photospherique^ 112. 

— shows A and B lines to be atmos 
pheric, 93. 

— solar photography, 52, 112. 
Jelinek, influence of sun-spots on 

the temperature of the earth, 172. 

Jevons, connection between sun- 
spots and commercial crises, 176, 
177. 

Jukowsky, rotation of sun, 143. 

Jupiter, influence upon sun-spots, 
158, 159. 

— satellites of, observed to determine 
tlie equation of light, 34, 35. 

KLINE, its reversal in faculse, 
109. 

reversal in sun-spots, 109. 

reversal in chromosphere and 

prominences, 109. 

double reversal of, 231. 

in spectro-heliographic work, 

234, 235. 
Kelvin, Lord, endurance of sun's 

heat if produced by the combustion 

of coal, 290. 
estimate of heat which would be 

produced by fall of planets upon 

the sun, 312, 313. 
on connection between sun-spots 

and magnetic storms, 169. 
Kew, photoheliograph, and photo- 
graphic record, 48, 49. 
Kirchhoff, map of solar spectrum, 

78, 134, 225, 257. 



INDEX. 



357 



Kirchhoff, spectroscopic work, 60, 80, 
81, 86. 

— theory of sun-spots, 178. 

LACAILLE, observations for solar 
parallax, 18. 
Lalande, theory of sun-spots, 178. 
Lambert, diminution of light at the 

limb of the sun, 279. 
Lane, estimate of the sun\«4 tempera- 
ture, 305. 

— law relating to the temperature of 
a contracting mass of gas, 315. 

Langley, bolometer and bolometric 
observations, 296-302. 

— color of the sun's limb compared 
with that of the center of the disk, 
282. 

— comparison between the intensity 
of solar radiation and that of metal 
in a Bessemer converter, 279, 309. 

— details of the solar surface {frontis- 
' piece)^ 103. 

— diminution of heat at the sun's 
limb, 281. 

— diminution of light at the sun's 
limb, 302. 

— effect of the sun's atmosphere and 
its changes upon the earth's tem- 
perature, 303. 

— extent of corona in eclipse of 1878, 
245, 246. 

— increase of solar radiation due to 
disturbance of the sun's surface, 
170. 

— observation of Mercury at the 
transit in 1878, 255. 

— solar eyepiece, 58. 

— spectroscopic observation of the 
sun's rotation, 100. 

— temperature of sun-spots, 170, 
note. 

— thermopile observations, 302. 

— true color of the sun, 284. 
Laplace, effect of the absorption of 

the atmosphere of the sun upon 
its brightness, 283. 



Latitude of sun-spots, Spocrer's dis- 
cov^ery, 157. 

Laugier, sun's equatorial accelera- 
tion, 138. 

Laussedat, horizontal photohelio- 
graph, 27. 

Le Ch atelier, on the temperature of 
the sun, 307. 

Lens, burning effect of, 308, 309. 

Leverrier, determination of the par- 
allax of the sun by means of 
planetary perturbations, 16. 

— perturbations of Mercury indicat- 
ing intra-Mercurial planets, 314. 

Liais, drawing of eclipse of 1857, 

240. 
Light of the sun, its total quantity 

in standard candle-power, 274, 324. 

its intensity, 278, 324. 

method of measuring, 

266, 267. 

— velocity of, used in determining 
the solar parallax, 16, 34, 35. 

Lindsay, Lord, expedition to Ascen- 
sion Island, 19. 

Liquefaction of gases, 329, 349. 

Lockyer, arrangement for studying 
the solar spectrum, 83. 

— connection between sun-spots and 
rainfall in India and Africa, 175. 

— discovery of the spectroscopic 
method of observing the chromo- 
sphere and prominences, 198-200. 

— discovery of the 1,474 line in the 
chromosphere spectrum, 257. 

— medal from the French Govern- 
ment, 199. 

— observation of the lines of hy- 
drogen in the corona spectrum, 
258. 

— theory as to the non-elementary 
character of so-called elements, 
90, 92. 

— use of annular slit for observing 
circumference of the sun, 213. 

— vibratinsr slit for observation of 
prominences, 200. 



358 



INDEX. 



Loomis, eft'ect of conjunctions of Ju- 
piter and Saturn upon sun-spot 
periodicity, 159. 

— relation between sun-spots and 
the aurora borealis, 164. 

Luminous radiations, groundlessly 
distinguished from thermal and 
chemical, 285. 

Lunar perturbations, as a means of 
determining the solar parallax, 32. 

MAGNETISM, terrestrial, period 
of disturbance corresponding 
with the sun-spot period, 162-164. 

aifected by solar paroxysms, 

120, 121, 164-170. 

Mars, observed as a means of deter- 
mining solar parallax, 17-21. 

Mass of the sun, 39, 40, 323. 

Maunder, on relation of faculse and 
prominences, 109. 

— on connection between sun-spots 
and terrestrial magnetism, 165, 
166. 

Maxwell, effect of temperature upon 
the viscosity of a gas, 331. 

Mechanical equivalent of heat, 312. 

Medal struck by the French Gov- 
ernment in honor of Janssen and 
Lockyer, 199. 

Meldrum, connection between sun- 
spots, cyclones, and rainfall, 173- 
175. 

Mercury (planet), influence on sun- 
spots, 158. 

— perturbations indicating intra- 
Mercurial matter, 314. 

— seen at transit on the background 
of the corona, 255. 

Mcrz helioscope, 56. 
Metallic prominences, 224. 
Metals, present in the sun, 87, 88. 
Meteors, possibly concerned in the 
formation of sun-spots, 160, 186. 

— regarded as the cause of the sun's 
equatorial acceleration, 141. 

Meteoric iron, helium present in, 346. 



Meteoric theory of the sun's heat, 
312, 313. 

Meudon, solar observatory, 52, 111. 

Michelson, determination of the ve- 
locity of light, 34. 

Mineral waters, helium present in, 
346. 

Motion in line of sight, spectroscop- 
ically observed, 97, 101. 

Mouchot, solar engine, 290. 

NASMYTH, willow-leaf structure 
of the sun's surface, 104. 
Newcomb, determination of the so- 
lar parallax, 36. 

— extent of the corona in the eclipse 
of 1878, 246. 

— speculations as to the age and 
duration of the sun, 317, 318. 

Nodes of the sun's equator, 144. 

OLSZEWSKI, attempt to liquefy 
helium, 349. 
Oppolzer, E., sun-spot theory, 190. 
Oxygen in the sun. Dr. H. Draper, 
94, 95. 

— A and B lines of, identified by 
Egoroff, 93. 

shown by Janssen to 

be atmospheric, 93. 

— spectra of, Schuster, 96. 

PALMIERI, supposed discovery of 
helium in lava, 345. 

Parallax, solar, defined, 12. 

determined by lunar perturba- 
tions, 32. 

determined by observations of 

Mars, 17-21. 

determined by planetary per- 
turbations, 32, 33. 

determined by transits of Ve- 
nus, 22-31. 

determined by the velocity of 

light, 34, 35. 

importance and difficulty of 

the problem, 10, 13. 



INDEX. 



559 



Parallax, solar, synopsis of methods 

for its determination, 15. 
values, according to different 

authorities, 20, 21, 22, 25, 31, 32, 

35, 36, 323. 
Peters, observations of sun-spots, 

123, 124. 

— volcanic theory of sun - spots, 
178. 

Petit, observation of the corona in 
1860, 256. 

Photographic observations of tran- 
sit of Venus, 25-31. 

Photographs of eclipses, 1860 — 195; 
1871—241, 248, 250; 1882—246, 
251; 1889—247, 253; 1893—248, 
255. 

Photography, solar, 47-53, 111, 112. 

Photoheliograph, the, 48. 

Photometric observations upon the 
corona, 252-256, 

Photosphere defined, 7. 

•— theories as to its nature, 110, 143, 
182, 187, 188, 335-337. 

Picard, observations for solar paral- 
lax, 18. 

Pickering, E. C, diminution of light 
at the sun's limb, 280, 283. 

— etfect of the sun's atmosphere 
upon its brightness, 284. 

— solar eyepiece, 58. 

Pitch of a sound altered by motion, 

98. 
Planetary perturbations as a means 

of determining solar parallax, 32, 

33. 

— infiuence upon sun-spots, 158- 
160. 

Planets, determination of their rela- 
tive distances, 16. 

Pogson, observation of eclipse of 
1868, 197. 

Polarization of the corona, 261. 

Polarizing eyepieces or helioscopes, 
57, 58. 

Position-angle of the sun's axis — 
table, 146. 



Potsdam, astrophysical observatory, 
308. 

Pouillet, estimate of the sun's tem- 
perature, 305. 

— measurement of the sun's heat, 
286, 291, 296. 

— pyrheliometer, 291. 

— temperature of space, 4. 
Powalky, computation of solar par- 
allax, 23. 

Princeton, Henry's thermopile ob- 
servations, 170, 302. 

— photographs of prominences and 
their spectrum, 209, 210, 231, 232. 

— spectroscopes used in the observ- 
atory, 69, 76. 

Prisms and prismatic spectrum, 61, 
65, 74. 

Problems in solar physics, 342, 343. 

Proctor, demonstration that the co- 
rona can not be due to the earth's 
atmosphere, 250. 

— velocity of matter ejected from 
the sun, 227. 

Projection of the sun's image on a 
screen, 43. 

Prominences or protuberances (so- 
lar), defined, 7. 

first named, 192. 

Pupin, M. I., coronoidal discharges, 
237. 

Purple tint of the nucleus of a sun- 
spot. 118. 

Pyrheliometer, 291, 292. 



Q 



UIESCErsT prominences, 218- 
221. 



RADIATION (total) of the sun, 
288-290, 324. 
Kamsay, identification of helium, 88, 

345. 
Eanyard, brightness of the inner 
corona, 255. 

— memoir on recent eclipses, 240. 

— synclinal structure of the corona, 
264. 



360 



INDEX. 



Kayet, observation of the eclipse of 
1868, 197. 

— discovery of helium line, A 7,065, 
344. 

Kayleigh, Lord, resolving power of 
spectroscopes, Q(). 

— discovery of Argon, 88, 345. 
Kecurrence of sun-spots at special 

points on the sun's surface, 150, 
334, 335. 
Keed, T., observation of duplicity of 
D3, 348. 

— photograph of prominence in C 
line, 232. 

Eellecting telescope with unsilvered 
mirror for observing the sun, 54. 

Reseau Photospherique^ Janssen, 
111, 112. 

Eespighi, depression of the chromo- 
sphere over a sun-spot, 216. 

— discovery of helium line, A 4,472, 
344. 

Reversal of bright lines to dark in 
the solar spectrum explained, 
81. 

— of dark lines to bright in a total 
eclipse, 82. 

— double, of C line, 209, 210. 

— double, of D lines in the chromo- 
sphere spectrum, 208. 

— double, of Hand K lines, 109, 231. 
Reversing stratum, of the solar at- 
mosphere tirst observed, 82, 83. 

its relation to the photosphere, 

325, 339.- 

Richer, observations for solar paral- 
lax, 18. 

Roemer, observations for solar par- 
allax, 18. 

"Rosa Ursina." Scheiner, 115. 

Rosetti, law of radiation and elec- 
tive temperature of the sun, 306. 

Ross, photometric observations upon 
the corona, 254. 

Rotation of the sun demonstrated 
by displacement of lines in the 
spectrum, 100, 101. 



Rotation of the snn, peculiar law 

of equatorial acceleration, 138, 

144. 
Rowland, H. A., concave grating 

spectroscope, 70. 

diffraction spectroscope, 67. 

list of elements identified in 

the sun, 87, 88. 
photographic map of the solar 

spectrum, 78, 79. 
Runge, spectrum of helium, 347, 348. 
Ruth erf urd, diffraction gratings, 66. 

— solar photography, 47. 

SATURN, influence on sun-spots, 
159. 

Schaeberle, J. M., theory of sun- 
spots, 186. 

photograph of eclipse of 1893, 

248, 255. 

theory of the corona, 271. 

Scheiner, Father C, discovery of 
sun-spots, 114. 

Scheiner, J., on solar temperature, 
308. 

Schmidt, theory of the constitution 
of the sun, 229. 

Schott, drawing of eclipse of 1869, 
245. 

Schuster, spectra of oxygen, 91. 

Schwabe, discovery of the periodici- 
ty of sun-spots, 151, 152. 

Secchi, classification of prominences, 
218. 

— drawing of eclipse of 1860, 241. 

— drawing of a sun-spot, 116. 

— estimate of the sun's temperature, 
305. 

— formation of detached cloud-like 
prominences, 221. 

— measurement of the variations of 
temperature at different parts of 
the sun's disk, 302. 

— photographs of the eclipse of 
1860, and inferences from them, 
195,196. 

— solar eyepiece, 58. 



INDEX. 



361 



Secchi, thermopile observations, 302. 

— theories of sun-spots, 181, 185. 
Sherman, observations at, 166, 167, 

205, 225. 

Siemens, W., theory of solar heat, 
318-320. 

Sierra, synonym for chromosphere, 
192. 

Silvered object-glass for viewing the 
sun, 54. 

Slitless spectroscope applied to the 
corona, 261-263. 

Smyth, records of rock-thermome- 
ters at Edinburgh, 173. 

Solar, constant, defined, 296. 

value of, 296, 324. 

Solar parallax (see Parallax). 

Soret, penetrating power of solar 
radiation, 310. 

Sources of solar heat, 311-315. 

Space, temperature of, 4. 

Spectral photometer, Vogel, 280. 

Spectra produced by prisms and 
gratings compared, 72, 

Spectrograph, the, 71, 76. 

Spectro-heliograph, 233-236. 

Spectroscope, analyzing and inte- 
grating, 72, 73. 

— automatic, 203. 

— described and discussed, 61-76. 

Spectrum, explanation of its for- 
mation in a spectroscope, 61, 
62. 

— difiraction, 67, 72. 

— lines, displacement of, by motion 
in the line of sight, 97-101. 

— of the corona, 257-261. 
a sun-spot, 132-136. 

— solar, discovery of the dark lines, 
59. 

early investigations as to the 

origin of the dark lines, 60. 
Kir chh oil's explanation of the 

dark lines, 81. 
maps or drawings of portions, 

77, 78, 100, 134, 135, 167, 209, 210, 

225, 231, 258. 



Spoerer, distribution of sun-spots, 
148, 149. 

— estimate of the sun's temperature, 
305. 

— formula for sun's ec^uatorial ac- 
celeration, 140. 

— peculiar law respecting latitude 
of spots, 157. 

— recurrence of spots at special 
points on the sun's surface, 150, 
334. 

Spots (see Sun-spots). 

Stannyan, Captain, discovers the 
chromosphere in 1706, 194. 

Stephan, law of radiation, 307. 

Stewart, Balfour, discussion of mag- 
netic observations at Kew, 154, 
164. 

uncertainty w^hether sun-spots 

raise or lower terrestrial tempera- 
ture, 172. 

Stone, calculation of solar parallax, 
23, 24. 

Stoney, J., suggests carbon as chief 
constituent of the photosphere, 
339. 

explanation of absence of he- 
lium from earth's atmosphere, 
349. 

Struve, brightness of the corona, 255. 

Sun-spots, cyclonic motion of, 126. 

depressions in tlie photosphere, 

128, 131. 

• development and dissolution, 

122, 123. 

dimensions, 126, 127. 

discovery in 1610, 114. 

distribution on the sun, 147- 

150. 

disturbances connected with 

them, 120, 122. 

duration, 119. 

eft'ects upon the earth, 161- 

177. 

periodicity, 151-161. 

spectrum, 132-136. 

Spoerer's law^ of latitudes. 157. 



362 . 



INDEX. 



Sun-spots, theories as to formation 
and nature of, 177-190. 

visible to the naked eye, 114, 

127, 128. 

Swan, spectroscopic observations, 00^ 
80. 

Symons, connection between sun- 
spots and rainfall, 175. 

Synclinal structure of the corona, 
204. 



TACCHINI, sun-spot of 1883, 128. 
Tarde, sidera Borhonica^ 115. 
Telespectroscope, 74, 76. 
Tempel, drawing of eclipse of 1860, 

242. 
Temperature of the sun, 305-309, 
324. 

the sun's center, 331. 

sun-spot, 170. 

— terrestrial as affected by sun- 
spots, 170-173. 

Tennant, Colonel, observation of 
eclipse of 1868, 197. 

Thickening and thinning of lines in 
the sun-spot spectrum, 133. 

Thermal rays, groundlessly distin- 
guished from luminous and chem- 
ical, 285. 

Thermopile, 170, 298, 302. 

Thollon, great map of the solar spec- 
trum, 79. 

— powerful spectroscopes, 65. 
Thomson, Sir W. (see Kelvin). 
Tisserand, formula for sun's equato- 
rial acceleration, 140. 

Transits of Venus, 22-31. 
Trouvelot, veiled spots, 136. 
Tupman, drawing of the eclipse of 
1871, 246. 

— work upon solar parallax, 24. 

ULLO A, Don. observation of eclipse 
of 1778, 194. 
Uraninite, a source of terrestrial he- 
lium, 346. 



VARIATIONS in solar radiation, 
304, 343. 
Vassenius, early observation of 

prominences, 193. 
Veeder, aurora period, 137. 

— relation between sun-spots and 
aurora, 165. 

Veiled sun-spots, Trouvelot, 136. 

Velocity of motion in solar promi- 
nences, 225, 226. 

Venus, influence upon sun-spots, 
158. 

— seen at transit before reaching 
the limb of the sun, 255. 

Vicaire, estimate of the sun's tem- 
perature, 306. 
Violle, actinometer, 294. 

— measure of the sun's heat, 293, 
294. 

— value of the solar constant, 296. 
Vogel, diminution of light at the 

sun's limb, 280, 281, 284. 

— effect of the sun's absorbing at- 
mosphere upon his total bright- 
ness, 284. 

— exposure slide for solar photog- 
raphy, 50. 

— spectral photometer, 280. 

— spectroscopic measurement of the 
sun's rotation, 100. 

WARRING, illustrations of the 
sun's attracting force on the 
earth, 41. 

Waterston, measure of solar heat, 
293. 

Wilna, photographic observations, 
52. 

Wilsing, determination of sun's rota- 
tion by observations of the faculoe, 
140. 

Wilson, Alexander, discovery that 
sun-spots are depressions in the 
sun's surface, 129. 

Wilson, W. E., and Gray, on tem- 
perature of the sun, 307. 

on radiation of sun-spots, 170. 



INDEX. 



3G3 



Wilson, \V. E., and Gray, diminu- 
tion of heat at sun's limb, 302. 

Winlock, horizontal photohelio- 
graph, 27. 

— annular slit for spectroscopic ob- 
servation of the prominences, 213. 

Wolf, magnetic variations following 
sun-spot period, 164, 

— periodicity of sun-spots and rela- 
tive numbers, 152-154. 

Wolf-Eayet stars, helium in their 

spectrum, 345. 
Wollaston, discovery of dark bands 

in the solar spectrum, 59. 

— measurement of the sun's light, 
275. 

YOUNGr, discovery of bright lines 
in the spectrum of the corona, 
250, 260. 

— disturbance of lines in sun-spot 
spectrum, 100, 135. 

^— double reversal of D lines, 135, 
208. 

— duplicity of corona line, 257. 

— examination of basic lines in the 
solar spectrum, 93. 

— experiment showing the black- 
ness of dark lines to be only rela- 
tive, 81. 



Young, observations on chromo- 
si^here lines at Sherman, 205. 

— observations on the corona at 
Denver in 1878, 239. 

— observations on remarkable 
prominences, 216, 217, 221, 223, 
225. 

— proposed explanation of equato- 
rial acceleration, 141, 142. 

— resolution of sun-spot spectrum, 
132, 133. 

— reversal of dark lines at the be- 
ginning of totality in the eclipse of 
1870, the reversing stratum, 82, 

— solar eruption followed by mag- 
netic disturbance, 166, 167, 225. 

— spectroscopic measurement of the 
sun's rotation, 100. 

— sun-spot spectrum, 132-134, 339. 

ZA^^TEDESCHI, development of 
the spectroscope, 60. 
Zollner, estimate of the sun's tem- 
perature, 305. 

— spectroscopic measurement of the 
sun's rotation, 100. 

— theory of sun-spots and liquid sur- 
face of the photosphere, 144, 182. 

— vibrating slit for observation of 
the prominences, 200. 



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ley, F. R. S., Professor of Geography in King's College, London. 
With Illustrations. 

HTHE STORY OF THE SOLAR SYSTEM. By 
1 G. F. Chambers, F.R.A. S. 



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New York : D. APPLETON & CO., 72 Fifth Avenue. 



D. APPLETON & CO.'S PUBLICATIONS. 



RICHARD A. PROCTOR'S WORKS. 

OTHER WORLDS THAN OURS: The Plurality 
of Worldsy Studied under the Light of Recent Scientific Re- 
searches. By Richard Anthony Proctor. With Illustra- 
tions, some colored. i2mo. Cloth, $1.75. 

Contents. — Introduction. — What the Earth teaches us.— What we learn from 
the Sun.— The Inferior Planets.— Mars, the Miniature of our Earth.— Jupiter, the 
Giant of the Solar System.— Saturn, the Ringed World. —Uranus and Neptune, the 
Arctic Planets.— The Moon and other Satellites.— Meteors and Comets : their Office 
in the Solar System.— Other Suns than Ours.— Of Minor Stars, and of the Distri- 
bution of Stars in Space.— The Nebulae: are they External Galaxies ? — Supervision 
and Control. 



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UR PLACE AMONG LNFLNITIES. A Series 

of Essays contrasting our Little Abode in Space and Time with 

the Infinities around us. To which are added Essays on the 

Jewish Sabbath and Astrology. i2mo. Cloth, $1.75. 

Contents. — Past and Future of the Earth. — Seeming Wastes in Nature.— New 
Theory of Life in other Worlds. — A Missing Comet.— The Lost Comet and its Me- 
teor Train.— Jupiter.— Saturn and its System.— A Giant Sun.— The Star Depths.— 
Star Gauging.— Saturn and the Sabbath of the Jews.— Thoughts on Astrology. 

qrHE EXPANSE OF HEAVEN. A Series of 

-^ Essays on the Wonders of the Firmament. i2mo. Cloth, 

$2.00. 

Contents.— A Dream that was not all a Dream.— The Sun.— The Queen of 
Night.— The Evening Star.— The Ruddy Planet.— Life in the Ruddy Planet.— The 
Prince of Planets. — Jupiter's Family of Moons.— The Ring-Girdled Planet.— New- 
ton and the Law of the Universe.— The Discovery of Two Giant Planets. — The 
Lost Comet. — Visitants from the Star Depths. — Whence come the Comets ? — The 
Comet Families of the Giant Planets.— The Earth's Journey through Showers.— 
How the Planets Grew.— Our Daily Light.— The Flight of Light.— A Cluster of 
Suns.— Worlds ruled by Colored Suns.— The King of Suns.— Four Orders of Suns. 
—The Depths of Space.— Charting the Star Depths.— The Star Depths Astir with 
Life.— The Drifting Stars.— The Milky Way. 

HE MOON : Her Motions, Aspect, Scenery, and Phys- 
ical Conditions, With Three Lunar Photographs, Map, and 
many Plates, Charts, etc. l2mo. Cloth, $2.00. 

Contents.— The Moon's Distance, Size, and Mass.— The Moon's Motions.— 
The Moon's Changes of Aspect, Rotation, Libration, etc.— Study of the Moon's 
Surface.— Lunar Celestial Phenomena.— Condition of the Moon's Surface.— Index 
to the Map of the Moon. 

IGHT SCIENCE FOR LEISURE HOURS. A 

Series of Familiar Essays on Scientific Subjects, Natural Phe- 
nomena, etc. i2mo. Cloth, $1.75. 



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